- Email: [email protected]
The stability and short-term fluctuation in serum oxytocinase activity in pregnancy
Int. J. Gynecol. Obstet., 1989,28: 331-335 International Federation of Gynecology and Obstetrics
331
The stability and short-term fluctuation oxytocinase activity in pregnancy A.A. Edebiri,
D.S. Mack, D.J. McDonald
in serum
and J. Philips
Departments of Obstetrics and Clinical Biochemistry, Rutherglen Maternity Hospital, Glasgow (UK) (Received September 3Oth, 1987) (Revised and accepted April 13th, 1988)
Abstract Maternal serum samples obtained from 123 normal pregnant women between 28 and 40 weeks were analysed for gestation oxytocinase (EC 3.4.11.3) activity using sbenzyl-1-cysteine-4-nitroanilide as substrate. Oxytocinase activity was found to be stable at varying temperatures, with a mean co-efficient of variation of 5.0%. The short-term flucutuation of oxytocinase activity had a mean coefficient of variation of 3.6%. There is no suggestion of a ‘pulsatile” release of oxytocinae from its placental origin. Keywords: Oxytocinase; Stability; Short-term fluctuations; Pulsatile release. Introduction Oxytocinase (EC 3.4.11.3) or cystine aminopeptidase has been identified in maternal serum [ 14,181. This substance inactivates oxytocin by cleaving the tyrosine-cystine link in this octapeptide, hence the derivation of its name. The same enzyme can cleave cystine from L-Cystine-bis-p-nitroanilide and it is, therefore, also known as cystine aminopeptidase. This substrate can be assayed quantitatively and has formed the biochemical basis for the asay of this enzyme. Oxytocinase is thought to be largely derived from the pla0020-7292/89/$3.50 0 1989 International Federation of Gynecology and Obstetrics Published and Printed in Ireland
centa [lo, 181, although other sites of enzyme production have been recognised [ 19,201. The exact role of oxytocinase in pregnancy remains uncertain. Various studies have been done to assess its physiological and pathological significance in pregnancy without conclusive results and in some situations provided conflicting evidence. Serum oxytocinase activity has been found to increase progressively from early pregnancy to maximum levels at term [14]. High levels of enzyme activity have been reported in twin pregnancy [14,20]. More recently, the high level of enzyme activity observed in amniotic fluid in early pregnancy was found to diminish exponentially with advancing gestation to minimum levels at term [19]. Other workers [2,3,6,22] have uniformly reported on the useful value of oxytocinase activity as a test of feto-placental function and in particular, intrauterine fetal growth retardation detection. Today, its place as a test of feto-placental function remains undecided despite the fact that serum oxytocinase activity is very simple to measure, rapid, accurate and easily reproducible without need for sophisticated equipment [ 1,4,23]. “Episodic”, “pulsatile” or “spurt” release of oxytocin from the neurohypophysis have been described [5,9,13]. More marked fluctuations of oxytocin have been observed at term and in labor but not in the midtrimester [13]. It is uncertain whether the Clinical and Clinical Research
332
Edebiri et al.
release of oxytocinase from the placenta mimics the pulsatile pattern of oxytocin release from the neurohypophysis. The present study was aimed at establishing the level of fluctuations and stability of oxytocinase activity in maternal serum. Materials and methods The study population comprised of pregnant women attending the Rutherglen Maternity Hospital, Glasgow (UK). The stability of oxytocinase activity was investigated by preparauing a large pool of serum samples (N = 118) obtained from pregnant women at 28-40 weeks of gestation. All these women had certain menstrual dates confirmed ultrasonically in early pregnancy. The serum pool was then split into three equal pools and each aliquoted into 14 specimens. One set of aliquots was stored at room temperature, the next at 4OC (refrigerator) and the third at - 20°C (deep freeze). The enzymic activity of one aliquot from each pool was measured on 14 successive days, covering a period of 3 working weeks. The activity of each pool was then plotted graphically every day. The short-term fluctuations in the levels of serum oxytocinase was studied by obtaining blood samples (IV = 28) from five volunteers. They were 16, 28, 30, 34 and 40 weeks pregnant, respectively. The patients were studied in the morning for 150 min. Clotted blood samples obtained at 30-min intervals over 150 min were separated usually within 3 h of collection, stored at - 20°C until analysed. The assay procedure used was essentially the method described elsewhere [4] and is summarised below. Fifty microliters of the patient’s serum sample was pipetted into a 10 ml polystyrene tube containing 1.75 ml of buffer solution. A blank solution was prepared by using 1.8 ml of buffer. The tubes were placed in a water bath at a temperature of 37OC and allowed to equilibrate to that temperature for 15 min. A Int J Gynecol Obstet 28
0.2-ml aliquot of substrate solution was added to each tube, mixed and incubated at 37 OC for 20 min. The reaction was terminated by adding 1 ml of trichloroacetic acid reagent. The tubes were mixed and centrifuged at 3000 rev./min for 10 min. The absorbance of each tube was read at 400 nm. The blank absorbance was subtracted from each reading and the oxytocinase activity of each sample read from the standard or calibration curve in units per liter. The precision of the assay was determined for low and high enzyme activity. The mean coefficient of variation for intra- and interassay estimations of the method were 2.8 and 5.0% respectively. The Student’s f-test was used for the analysis of statistical data. Results The daily fluctuations of serum oxytocinase levels at varying temperatures over a 14day period of measurements are shown in Fig. 1. There would appear to be no substantial loss in enzymic activity for 18 days (i.e. 3
600
t
---
Room
-
L'C
,.mperatur*
100 t 01
2
4
6
6
10
12
11
DAYS
Fig. 1. Daily fluctuations of serum oxytocinase activity at varying temperatures. The coefficients of variation at room temperature, - 4OC and - 20°C were 5.2’70, 5.0% and 5.0070, respectively. There was no statistical difference between the mean values of oxytocinase activity for room temperature and - 20% (P> 0.05).
Serum oxytocinase activity in pregnancy
working weeks) when serum is stored at room temperature, 4OC (refrigerator) and at - 2o”c. The mean f standard deviation (SD.) for oxytocinase values after 14 days measurement at room temperature, 4OC and - 20°C were 299 2 15.7, 303 f 15.4 and 305 f 15.4 units/l, respectively and their corresponding coefficients of variation were 5.2, 5.0 and 5.0070, respectively. There was no statistical difference between the mean values for room temperature and - 2o”c (P > 0.05). The short-term fluctuations of serum oxytocinase levels over 150 min are depicted in Fig. 2. The five patients A, B, C, D and E demonstrate remarkably steady enzyme levels over 150 min. The mean -r- S.D. oxytocinase levels for patients A, B, C, D and E were 391 f 11,221 -+ 6, 167 f 8, 146 f 6and 78 f 3 units/l, respectively. The corresponding coefficients of variation in percentage were 2.8, 2.7, 4.8, 4.1 and 3.8 with a mean of 3.6%, a variability within 5070, i.e. error of the method.
0 Fig. 2.
30
60 Time
9C 120 In minutes
150
180
Short-term fluctuations in serum oxytocinase levels patients A, B, C, D and E at different gestational ages. The oxytocinase activity over 150 min in all patients is remarkably steady, with a mean coefficient of variation of only 3.6%. The figures in parentheses represent the gestational age (weeks) of the pregnancies. in five
333
Discussion Significant measurable serum oxytocinase activity after 3 months of storage at 4OC (refrigerator temperature) has been demonstrated [2]. The present study could not find any statistically significant difference in the mean oxytocinase levels after 18 days of storage between room temperature, 4OC and - 20°C. Serum oxytocinase, being very stable, therefore has good potential for use in areas where there is no constant electricity supply and such areas utilise a centralised laboratory service. This study did not show any remarkable fluctuations in serial oxytocinase over 150 min in early and late second and third trimesters of pregnancies. It should have been desirable to estimate the oxytocin levels simultaneously along with serial oxytocinase measurements. The lack of radioimmunoassay facility in this center was the major constraint in this direction. Nevertheless, there is the suggestion that serum oxytocinase levels do not fluctuate dramatically over a short time interval and indeed is appreciably steady in the same subject within the study period. Furthermore, the mean coefficient of variation of only 3.6% implies a steady release of oxytocinase into the circulation rather than the occasional “spurt”, “episodic” or “pulsatile” release noted with oxytocin. The steady oxytocinase level may owe as much to its slow removal from the circulation (halflife of 72 h) [8] as to an even rate of input. A gradual rise in oxytocin [ 131 and oxytocinase [14] levels from early pregnancy towards term have been observed. However, no obvious relationships have been found between oxytocin, oxytocinase and uterine contractions [7,8,13]. The paradoxical peak serum oxytocinase and nadir in amniotic fluid [19] oxytocinase activity in term pregnancy suggests an inverse relationship between the enzyme levels in serum and amniotic fluid. The dynamics of transfer between serum and amniotic fluid oxytocinase will require further elucidation. The roles of this enzyme in Clinical and Clinical Research
334
Edebiri et al.
serum and amniotic fluid may well be different in the pregnancy and labor situations . It is surmised that oxytocinase activity should mirror oxytocin levels if indeed, the main physiological function of oxytocinase was to neutralise oxytocin action and the pattern of release ought to be similar. Contrarily, the results of this study indicate an independent level of fluctuation of oxytocinase activity to that of oxytocin. Serum oxytocinase has been shown to possess some vasopressinase [ 171and angiotensinase [21] activity. The oxytocinase action of this enzyme may well be the least important of its function. Although still in common use, the hormonal tests of feto-placental function have a poor discriminatory value between physiological and pathological pregnancy [ 161. We have found that oxytocinase levels fluctuate less dramatically then human placental lactogen with a coefficient of variation of 5-10% [15] plasma estriol with 14% [12] and urinary estriol 18% [ll]. The time-to-time variation of serum oxytocinase is important as a criterion for choice as a test. Thus, if the variation is very large, interpretation of the test is exceedingly difficult as have been found with urinary and plasma estrogens with coefficients of variation of 14-18%. The coefficient of variation of serum oxytocinase of 2.7-5.2070 has a useful practical application in that it fulfils the minimum criterion required of any placental function test. The rather low variability makes the timing of blood sample collection unnecessary. References 1
2
3
Babuna C, Yenen E: Enzymatic determination of placental function: a rapid method. Am J Obstet Gynecol 95: 925, 1966. Blunt A: The value of plasma oxytocinase in the assessment of fetal-placental function. Aust NZ J Obstet Gynaecol I: 37, 1971. Briet JW, Hoorn RKJ: The use of human placental lactogen oxytocinase and oestriol in twin pregnancy with intrauterine growth retardation. Eur J Obstet Gynecol Reprod BiolZ3: 7, 1982.
Int J Gynecol Obstet 28
6
8
9
10
11 12
16
17
18
19
20
Durham BH: Spectrophotometric end-point for assay of serum cystyl-aminopeptidase in pregnancy. Clin Chem 22: 79, 1976. Gibbens D, Chard T: Observation on maternal oxytocin release during human labour and the effect of intravenous alcohol administration. Am J Obstet Gynecol 126: 243, 1976. Gopalaswamy G, Balasubramaniam N, Kanagasabapathy AS: Gystyl aminopeptidase in maternal serum for the antenatal recognition of fetal growth retardation. Aust NZ J Obstet Gynaecol23: 79, 1983. Hull MGR, Monro PP, Ellis BW: The relation between plasma unconjugated oestriol and cystine aminopeptidase concentrations and uterine contractions. Br J Obstet Gynaecol86: 180,1979. Hurry DJ, Tovey JE, Robinson DA, Beynon CL, Cooper K: Cystine aminopeptidase in normal and complicated pregnancies. J Obstet Gynaecol Br Commonw 79: 788, 1972. Jacobs HS: Hypothalamus and pituitary glands oxytocin. In Clinical Physiology in Obstetrics (eds FE Hytten, G Chamberlain), p 394. Blackwell Scientific Publications, Oxford, 1980. James NT: Histochemical demonstration of oxytocinase in human placenta. Nature210: 1276,1%6. Klopper A, Wilson G, Cooke I: Variability of urinary steroid excretion. J Endocrinol43: 295, 1969. Klopper A, Wilson G, Masson G: The variability of plasma hormone levels in late pregnancy. In Hormonal Investigations in Human Pregnancy. (ed R Scholer), p 77. Sepe, Paris, 1974. Otsuki Y, Yamaji K, Fujita M, Takagi T, Tanizawa 0: Serial plasma oxytocin levels during pregnancy and labour. Acta Obstet Gynecol Stand 62: IS, 1983. Page EN, Titus MA, Mohun G, Glendening MB: The origin and distribution of oxytocinase. Am J Obstet Gynecol 82: 1090, l%l. Pavlou C, Chard T, Letchworth AT: Circulating levels of human chorionic somatomammotrophin in late pregnancy: disappearance from the circulation after delivery, variation during labour and circadian variation. Br J Obstet Gynaecol Commonw 79: 629,1972. Pearson JF: Monitoring high-risk pregnancy. In Recent Advances in Obstetrics and Gynaecology No. 14 (ed J Bonnar), p 3. Churchill Livingstone, Edinburgh, 1982. Rosenbloom AA, Sach J, Fisher DA: The circulating vasopressinase of pregnancy: species comparison with radioimmunoassay. Am J Obstet Gynecol121: 316, 1975. Roy AC, Karim SMM: Review: significance of inhibition by prostaglandins and cyclic GMP of oxytocinase activity in human pregnancy and labour. Prostaglandins 25: 55, 1983. Roy AC, Kottegoda SR, Viegas OAC, Ratnam SS: Oxytocinase activity in human amniotic fluid and relationship to gestational age. Obstet Gynecol68: 614, 1986. Ryden G: Cystine aminopeptidase and oxytocinase activity in pregnancy. A comparative study in human and rat tissues. Acta Obstet Gynecol Stand 45: (Suppl3) 1, 1966.
Serum oxytocinase activity in pregnancy
21
22
23
Sjoholm 1, Yman L: Degradation of Oxytocin, LysineVasopressin, Angiotensin II and Angiotensin-II-amide by Oxytocinase (Cystine-aminopeptidase). Act Pharm Suecica 4: 65, 1967. Spellacy WN, Usategui-Ciomez M, Fernandez-De Castro A: Plasma human placental lactogen, oxytocinase and placental phosphatase in normal and toxemic pregnancies. Am J Obstet Gynecol127: 10, 1977. Wood SM, Wright PM: Placental function testing using finger-prick samples. A micro-method for plasma oxytocinase. Br J Obstet Gynaecol82: 844, 1975.
335
Address for reprints: A.A. Edebiri Department of Obstetrics und Gyaaecology Bayer0 University PMB3011 Knno Nigeria
Clinical and Clinical Research
331
The stability and short-term fluctuation oxytocinase activity in pregnancy A.A. Edebiri,
D.S. Mack, D.J. McDonald
in serum
and J. Philips
Departments of Obstetrics and Clinical Biochemistry, Rutherglen Maternity Hospital, Glasgow (UK) (Received September 3Oth, 1987) (Revised and accepted April 13th, 1988)
Abstract Maternal serum samples obtained from 123 normal pregnant women between 28 and 40 weeks were analysed for gestation oxytocinase (EC 3.4.11.3) activity using sbenzyl-1-cysteine-4-nitroanilide as substrate. Oxytocinase activity was found to be stable at varying temperatures, with a mean co-efficient of variation of 5.0%. The short-term flucutuation of oxytocinase activity had a mean coefficient of variation of 3.6%. There is no suggestion of a ‘pulsatile” release of oxytocinae from its placental origin. Keywords: Oxytocinase; Stability; Short-term fluctuations; Pulsatile release. Introduction Oxytocinase (EC 3.4.11.3) or cystine aminopeptidase has been identified in maternal serum [ 14,181. This substance inactivates oxytocin by cleaving the tyrosine-cystine link in this octapeptide, hence the derivation of its name. The same enzyme can cleave cystine from L-Cystine-bis-p-nitroanilide and it is, therefore, also known as cystine aminopeptidase. This substrate can be assayed quantitatively and has formed the biochemical basis for the asay of this enzyme. Oxytocinase is thought to be largely derived from the pla0020-7292/89/$3.50 0 1989 International Federation of Gynecology and Obstetrics Published and Printed in Ireland
centa [lo, 181, although other sites of enzyme production have been recognised [ 19,201. The exact role of oxytocinase in pregnancy remains uncertain. Various studies have been done to assess its physiological and pathological significance in pregnancy without conclusive results and in some situations provided conflicting evidence. Serum oxytocinase activity has been found to increase progressively from early pregnancy to maximum levels at term [14]. High levels of enzyme activity have been reported in twin pregnancy [14,20]. More recently, the high level of enzyme activity observed in amniotic fluid in early pregnancy was found to diminish exponentially with advancing gestation to minimum levels at term [19]. Other workers [2,3,6,22] have uniformly reported on the useful value of oxytocinase activity as a test of feto-placental function and in particular, intrauterine fetal growth retardation detection. Today, its place as a test of feto-placental function remains undecided despite the fact that serum oxytocinase activity is very simple to measure, rapid, accurate and easily reproducible without need for sophisticated equipment [ 1,4,23]. “Episodic”, “pulsatile” or “spurt” release of oxytocin from the neurohypophysis have been described [5,9,13]. More marked fluctuations of oxytocin have been observed at term and in labor but not in the midtrimester [13]. It is uncertain whether the Clinical and Clinical Research
332
Edebiri et al.
release of oxytocinase from the placenta mimics the pulsatile pattern of oxytocin release from the neurohypophysis. The present study was aimed at establishing the level of fluctuations and stability of oxytocinase activity in maternal serum. Materials and methods The study population comprised of pregnant women attending the Rutherglen Maternity Hospital, Glasgow (UK). The stability of oxytocinase activity was investigated by preparauing a large pool of serum samples (N = 118) obtained from pregnant women at 28-40 weeks of gestation. All these women had certain menstrual dates confirmed ultrasonically in early pregnancy. The serum pool was then split into three equal pools and each aliquoted into 14 specimens. One set of aliquots was stored at room temperature, the next at 4OC (refrigerator) and the third at - 20°C (deep freeze). The enzymic activity of one aliquot from each pool was measured on 14 successive days, covering a period of 3 working weeks. The activity of each pool was then plotted graphically every day. The short-term fluctuations in the levels of serum oxytocinase was studied by obtaining blood samples (IV = 28) from five volunteers. They were 16, 28, 30, 34 and 40 weeks pregnant, respectively. The patients were studied in the morning for 150 min. Clotted blood samples obtained at 30-min intervals over 150 min were separated usually within 3 h of collection, stored at - 20°C until analysed. The assay procedure used was essentially the method described elsewhere [4] and is summarised below. Fifty microliters of the patient’s serum sample was pipetted into a 10 ml polystyrene tube containing 1.75 ml of buffer solution. A blank solution was prepared by using 1.8 ml of buffer. The tubes were placed in a water bath at a temperature of 37OC and allowed to equilibrate to that temperature for 15 min. A Int J Gynecol Obstet 28
0.2-ml aliquot of substrate solution was added to each tube, mixed and incubated at 37 OC for 20 min. The reaction was terminated by adding 1 ml of trichloroacetic acid reagent. The tubes were mixed and centrifuged at 3000 rev./min for 10 min. The absorbance of each tube was read at 400 nm. The blank absorbance was subtracted from each reading and the oxytocinase activity of each sample read from the standard or calibration curve in units per liter. The precision of the assay was determined for low and high enzyme activity. The mean coefficient of variation for intra- and interassay estimations of the method were 2.8 and 5.0% respectively. The Student’s f-test was used for the analysis of statistical data. Results The daily fluctuations of serum oxytocinase levels at varying temperatures over a 14day period of measurements are shown in Fig. 1. There would appear to be no substantial loss in enzymic activity for 18 days (i.e. 3
600
t
---
Room
-
L'C
,.mperatur*
100 t 01
2
4
6
6
10
12
11
DAYS
Fig. 1. Daily fluctuations of serum oxytocinase activity at varying temperatures. The coefficients of variation at room temperature, - 4OC and - 20°C were 5.2’70, 5.0% and 5.0070, respectively. There was no statistical difference between the mean values of oxytocinase activity for room temperature and - 20% (P> 0.05).
Serum oxytocinase activity in pregnancy
working weeks) when serum is stored at room temperature, 4OC (refrigerator) and at - 2o”c. The mean f standard deviation (SD.) for oxytocinase values after 14 days measurement at room temperature, 4OC and - 20°C were 299 2 15.7, 303 f 15.4 and 305 f 15.4 units/l, respectively and their corresponding coefficients of variation were 5.2, 5.0 and 5.0070, respectively. There was no statistical difference between the mean values for room temperature and - 2o”c (P > 0.05). The short-term fluctuations of serum oxytocinase levels over 150 min are depicted in Fig. 2. The five patients A, B, C, D and E demonstrate remarkably steady enzyme levels over 150 min. The mean -r- S.D. oxytocinase levels for patients A, B, C, D and E were 391 f 11,221 -+ 6, 167 f 8, 146 f 6and 78 f 3 units/l, respectively. The corresponding coefficients of variation in percentage were 2.8, 2.7, 4.8, 4.1 and 3.8 with a mean of 3.6%, a variability within 5070, i.e. error of the method.
0 Fig. 2.
30
60 Time
9C 120 In minutes
150
180
Short-term fluctuations in serum oxytocinase levels patients A, B, C, D and E at different gestational ages. The oxytocinase activity over 150 min in all patients is remarkably steady, with a mean coefficient of variation of only 3.6%. The figures in parentheses represent the gestational age (weeks) of the pregnancies. in five
333
Discussion Significant measurable serum oxytocinase activity after 3 months of storage at 4OC (refrigerator temperature) has been demonstrated [2]. The present study could not find any statistically significant difference in the mean oxytocinase levels after 18 days of storage between room temperature, 4OC and - 20°C. Serum oxytocinase, being very stable, therefore has good potential for use in areas where there is no constant electricity supply and such areas utilise a centralised laboratory service. This study did not show any remarkable fluctuations in serial oxytocinase over 150 min in early and late second and third trimesters of pregnancies. It should have been desirable to estimate the oxytocin levels simultaneously along with serial oxytocinase measurements. The lack of radioimmunoassay facility in this center was the major constraint in this direction. Nevertheless, there is the suggestion that serum oxytocinase levels do not fluctuate dramatically over a short time interval and indeed is appreciably steady in the same subject within the study period. Furthermore, the mean coefficient of variation of only 3.6% implies a steady release of oxytocinase into the circulation rather than the occasional “spurt”, “episodic” or “pulsatile” release noted with oxytocin. The steady oxytocinase level may owe as much to its slow removal from the circulation (halflife of 72 h) [8] as to an even rate of input. A gradual rise in oxytocin [ 131 and oxytocinase [14] levels from early pregnancy towards term have been observed. However, no obvious relationships have been found between oxytocin, oxytocinase and uterine contractions [7,8,13]. The paradoxical peak serum oxytocinase and nadir in amniotic fluid [19] oxytocinase activity in term pregnancy suggests an inverse relationship between the enzyme levels in serum and amniotic fluid. The dynamics of transfer between serum and amniotic fluid oxytocinase will require further elucidation. The roles of this enzyme in Clinical and Clinical Research
334
Edebiri et al.
serum and amniotic fluid may well be different in the pregnancy and labor situations . It is surmised that oxytocinase activity should mirror oxytocin levels if indeed, the main physiological function of oxytocinase was to neutralise oxytocin action and the pattern of release ought to be similar. Contrarily, the results of this study indicate an independent level of fluctuation of oxytocinase activity to that of oxytocin. Serum oxytocinase has been shown to possess some vasopressinase [ 171and angiotensinase [21] activity. The oxytocinase action of this enzyme may well be the least important of its function. Although still in common use, the hormonal tests of feto-placental function have a poor discriminatory value between physiological and pathological pregnancy [ 161. We have found that oxytocinase levels fluctuate less dramatically then human placental lactogen with a coefficient of variation of 5-10% [15] plasma estriol with 14% [12] and urinary estriol 18% [ll]. The time-to-time variation of serum oxytocinase is important as a criterion for choice as a test. Thus, if the variation is very large, interpretation of the test is exceedingly difficult as have been found with urinary and plasma estrogens with coefficients of variation of 14-18%. The coefficient of variation of serum oxytocinase of 2.7-5.2070 has a useful practical application in that it fulfils the minimum criterion required of any placental function test. The rather low variability makes the timing of blood sample collection unnecessary. References 1
2
3
Babuna C, Yenen E: Enzymatic determination of placental function: a rapid method. Am J Obstet Gynecol 95: 925, 1966. Blunt A: The value of plasma oxytocinase in the assessment of fetal-placental function. Aust NZ J Obstet Gynaecol I: 37, 1971. Briet JW, Hoorn RKJ: The use of human placental lactogen oxytocinase and oestriol in twin pregnancy with intrauterine growth retardation. Eur J Obstet Gynecol Reprod BiolZ3: 7, 1982.
Int J Gynecol Obstet 28
6
8
9
10
11 12
16
17
18
19
20
Durham BH: Spectrophotometric end-point for assay of serum cystyl-aminopeptidase in pregnancy. Clin Chem 22: 79, 1976. Gibbens D, Chard T: Observation on maternal oxytocin release during human labour and the effect of intravenous alcohol administration. Am J Obstet Gynecol 126: 243, 1976. Gopalaswamy G, Balasubramaniam N, Kanagasabapathy AS: Gystyl aminopeptidase in maternal serum for the antenatal recognition of fetal growth retardation. Aust NZ J Obstet Gynaecol23: 79, 1983. Hull MGR, Monro PP, Ellis BW: The relation between plasma unconjugated oestriol and cystine aminopeptidase concentrations and uterine contractions. Br J Obstet Gynaecol86: 180,1979. Hurry DJ, Tovey JE, Robinson DA, Beynon CL, Cooper K: Cystine aminopeptidase in normal and complicated pregnancies. J Obstet Gynaecol Br Commonw 79: 788, 1972. Jacobs HS: Hypothalamus and pituitary glands oxytocin. In Clinical Physiology in Obstetrics (eds FE Hytten, G Chamberlain), p 394. Blackwell Scientific Publications, Oxford, 1980. James NT: Histochemical demonstration of oxytocinase in human placenta. Nature210: 1276,1%6. Klopper A, Wilson G, Cooke I: Variability of urinary steroid excretion. J Endocrinol43: 295, 1969. Klopper A, Wilson G, Masson G: The variability of plasma hormone levels in late pregnancy. In Hormonal Investigations in Human Pregnancy. (ed R Scholer), p 77. Sepe, Paris, 1974. Otsuki Y, Yamaji K, Fujita M, Takagi T, Tanizawa 0: Serial plasma oxytocin levels during pregnancy and labour. Acta Obstet Gynecol Stand 62: IS, 1983. Page EN, Titus MA, Mohun G, Glendening MB: The origin and distribution of oxytocinase. Am J Obstet Gynecol 82: 1090, l%l. Pavlou C, Chard T, Letchworth AT: Circulating levels of human chorionic somatomammotrophin in late pregnancy: disappearance from the circulation after delivery, variation during labour and circadian variation. Br J Obstet Gynaecol Commonw 79: 629,1972. Pearson JF: Monitoring high-risk pregnancy. In Recent Advances in Obstetrics and Gynaecology No. 14 (ed J Bonnar), p 3. Churchill Livingstone, Edinburgh, 1982. Rosenbloom AA, Sach J, Fisher DA: The circulating vasopressinase of pregnancy: species comparison with radioimmunoassay. Am J Obstet Gynecol121: 316, 1975. Roy AC, Karim SMM: Review: significance of inhibition by prostaglandins and cyclic GMP of oxytocinase activity in human pregnancy and labour. Prostaglandins 25: 55, 1983. Roy AC, Kottegoda SR, Viegas OAC, Ratnam SS: Oxytocinase activity in human amniotic fluid and relationship to gestational age. Obstet Gynecol68: 614, 1986. Ryden G: Cystine aminopeptidase and oxytocinase activity in pregnancy. A comparative study in human and rat tissues. Acta Obstet Gynecol Stand 45: (Suppl3) 1, 1966.
Serum oxytocinase activity in pregnancy
21
22
23
Sjoholm 1, Yman L: Degradation of Oxytocin, LysineVasopressin, Angiotensin II and Angiotensin-II-amide by Oxytocinase (Cystine-aminopeptidase). Act Pharm Suecica 4: 65, 1967. Spellacy WN, Usategui-Ciomez M, Fernandez-De Castro A: Plasma human placental lactogen, oxytocinase and placental phosphatase in normal and toxemic pregnancies. Am J Obstet Gynecol127: 10, 1977. Wood SM, Wright PM: Placental function testing using finger-prick samples. A micro-method for plasma oxytocinase. Br J Obstet Gynaecol82: 844, 1975.
335
Address for reprints: A.A. Edebiri Department of Obstetrics und Gyaaecology Bayer0 University PMB3011 Knno Nigeria
Clinical and Clinical Research