A study of plasma free amino acid levels. VIII. The relationship between plasma amino acid levels and I.Q. test performance

A Study of Plasma Free Amino Acid Levels. VIII. The Relationship Retween Plasma Amino Acid Levels and I.Q. Test Performance Marvin

D. Armstrong,

Robert

Fasting plasma free amino acid levels and standardized 1.0. scores for children participating in the F&s Longitudinal Study of growth and development were examined for possible relationships. Two partial replication samples were formed. One sample was composed of the first amino acid analysis made for a subject

B. McCall,

and Uwe

Stave

and the 1.0. score obtained nearest to it in time; the second sample consisted of the last analysis and the 1.0. test nearest to it. After age was linearly regressed from the amino acid levels, there were nontrivial correlations between amino acid level and I.Q. in both samples only in the case of tryptophan for boys and glycine for girls.

A

MINO ACIDS have many functions in brain tissue in addition to serving as precursors for proteins. The importance of glutamic acid in brain metabolism is well known.’ y-Aminobutyric acid, formed from glutamic acid, is known to act as an inhibitory substance on the synaptic transmission of neuronal implilses,2 it has been suggested that taurine may have a similar function,) and it has been proposed that glycine, glutamic acid, and aspartic acid are neurotransmittors in the central nervous system.4 Tyrosine serves as a precursor for dopa, which, in turn, is a precursor for dopamine and norepinephrine, important neurohormones. 5 Similarly, tryptophan is converted to 5-hydroxytryptophan, which is converted to serotonin, another neurohormone.6 There are several inherited abnormalities in the metabolism of single amino acids that feature greatly increased plasma levels of the amino acid involved and which are characterized by mental retardation and neurologic abnormalities, although the details of the pathogenesis are unknown. In addition, markedly altered levels of blood amino acids are characteristic of children with kwashiorkor, a condition in which a general retardation of intellectual development is observed.7 Schwerin et al? presented evidence that increasing the blood glutamine level of mice led to an increased glutamine content of the brain. More recently Fernstrom and Wurtmanv showed that the intraperitoneal injection of tryptophan into rats resulted in an increased amount of tryptophan and serotonin in the brain. These observations raise the possibility that blood levels of the individual amino acids might affect the brain levels, and because of the importance of amino acids in brain metabolism they might have an influence on mental ability. A longitudinal study of the fasting levels of the plasma free amino acids

From the Fels Research Insritute. Yellow Springs, Ohio 45387. Receivedfor publication April 24, 1973. Supported in part by OSPHS Grants HD-1202. MU-2278, HD-4160, and FR-0222. Marvin D. Armstrong, Ph.D.: Senior Scientisr, Fek Research Instirute, Yellow Springs, Ohio. Robert B. McCall, Ph.D.: Senior Scienlist, Feis Research Ins&are, Yellow Springs, Ohio. Uwe Stave, M.D.: Senior Scieniist, Fels Research Insii~aie, Ye/low Springs. Ohio. Q 1973 by Grme dt Stratton, Inr. Metabolism. Vol. 22. No. 11 (November), 1973

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ARMSTRONG,

MCCALL,

AND

STAVE

of children is being conducted in this laboratory; the results of I.Q. tests made on the subjects are also in the records. Therefore, an exploratory examination of the possibility that fasting plasma levels of the amino acids might have some relationship to mental functioning was carried out. Correlations were sought between amino acid levels and I.Q. test scores using two partial-replication samples of boys and girls separately; age was linearly regressed from the amino acid levels to adjust for changes that occur during growth.” Nontrivial correlations between amino acid level and I.Q. in both samples were found only in the case of tryptophan for boys and glycine for girls. The subjects were children of urban and rural, lower-to-upper middle class families of northwest European origin. As a group, they have higher than average 1.Q.s (x = 117) but normal variability (SD = 16.9). All blood samples were taken after an overnight fast. Details of the conditions and procedures used for collecting blood, preparing plasma filtrates for analysis and doing amino acid analyses and precautions taken to assure that subjects had not actually eaten before samples were taken have been reported elsewhere.” In many cases, more than one analysis had been made for the same subject, so two replicate sets of data were assembled. One set used the first amino acid analysis made on a subject and the I.Q. test score obtained nearest in time to that analysis. The other data set contained the most recent analysis made for a subject and another I.Q. test score assessed nearest in time to this chemical analysis. Of the 98 subjects available, 56 contributed data to both data sets. Thus, these partial replicate groups control for chance effects due to measurement error but only partially control for stable individual differences produced by other factors. For each of these subjects, the Fels Longitudinal Study files were searched for an I.Q. assessment made close in time to the biochemical assay. Both the chemical and I.Q. assessments were made without knowledge of the results of the other. If a child had a Standford-Binet (SB), Wechsler Intelligence Scale for Children (WISC), Wechsler Adult Intelligence Scale (WAIS), or WechslerBellevue (WB) I.Q. assessment, he was included in the sample of children for all analyses involving the I.Q. variable. The sample characteristics are summarized in Table 1. The I.Q. scores at each age wnnm each test for all children in the Fels sample (not limited to this group having chemical data) were converted to standard scores (mean = 0, SD = 1). The average N for these standardizations within each test within each age was 151. In this way, each child in the biochemistry sample having some I.Q. test had a standardized score on that I.Q. test that was statistically comparable across age and tests. It should be noted that statistical comparability among different tests does not guarantee psychologic comparability. Regression analysis of data from a larger sample of children (76 boys and 60 girls), 6-18 yr of age, showed that 15 amino acids had significant relationships with age for boys and seven for girlsi Since visual inspection of scatterplots failed to reveal any curvilinearity, age was linearly regressed out of all the amino acid values. These regressions were calculated separately for the present two data sets, using only the data from the subjects in these samples (Table 1) rather than from the larger group cited above. There was only a small and

PLASMA AMINO

ACIDS.

~III.

RELATIONSHIP

1.a.

WITH

Table 1. Characteristics

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of the SamDIes’ Girl5

BOYS

FirstAnalysis

Second

N = 49

Age (mo) At time of chemical analysis At time of 1.0. test Difference between chemical

Analysis

N=31

144.2

f

31.2

125.5 f

165.8

f

40.8

140.6

21.7

f

12.7

Fwt

Analpis

N = 43

Second

Analyas

N=31

31.8

135.6

f

32.7

115.9 f

32.4

* 40.7

157.9

It 38.9

130.8 *

36.8

15.6 *

12.3

22.8

f

11.7

lS.l’f

8.8

and I.Q. Number of subjects having each I.Q. test SBt

2

2

2

6

WISC

26

19

24

21

WAIS

21

8

17

3

0

2

0

1

W8

‘Included in both analyses were 39 boys and 26 girls. t Abbreviations are listed in the text.

insignificant relationship between age and I.Q. in these samples (see Table 2), so age was not regressed out of I.Q. Thus the correlations with I.Q. to be presented are part correlations, i.e., the correlation between an amino acid level and I.Q. with the effect of age on the amino acid level is, statistically controlled. The results of these analyses are given in Table 2. If the amount of an amino acid in the blood were to have a physiologically important relation to mental performance, it might be expected that a significant correlation would have appeared in each sample and with both sexes. This did not occur for any of the amino acids. However, there were statistically nontrivial part correlations for both sets of analyses only in the case of tryptophan (0,24,0.41) for boys and glycine (0.36, 0.34) for girls. A mechanism relating increased amounts of either amino acid to mental ability is not obvious. While these relationships are small, the only previously reported correlations between a blood constituent and I.Q. known to the authors have been for uric acid (I^ = 0.0812 and 0.10i3) and cholesterol (r = 0.1613). Two of these correlations were statistically “significant” because of large samples. The probability of obtaining a replicated result for at least one of the 24 amino acids for one of the sexes by chance cannot be determined precisely because the amino acid levels are not independent and because the two sampies were only partially independent (aIthough the assessment procedures were independent). However, given sample independence, the probability falls between 0.01 and 0.21. If the amino acids were perfectly correlated, the probability of a significant replication (a = 0.10) in an otherwise random system would be 0.01. If they were totally independent of one another the probability of at least one repficated result by chance would be 1 - (.99)24 = 0.21. The true probability must fall somewhere between these values. We speculate that it may be below 0.10, since nearly half of the intercorrelations among the amino acids were significant in our sample, some reaching high levels (e.g., > 0.80). Nevertheless, in view of this unavoidable ambiguity, these results should be viewed cautiously and may suggest hypotheses for future testing. The lack of correlation between plasma glutamine levels and I.Q. should be

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MCCALL,

AND STAVE

Table 2. Part Correlations Between Fasting Plasma Amino Acid Levels (Age Controlled) and Standardized I.Q. Score

FirstAnalysts

Last Analysts

Fwst Analyses

N

I

N

I

N

Taurine

48

0.06

31

0.23

43

0.04

I

Last Analysts N

0.19

31

0.06

28

I

-0.07

Hydroxyproline

47

28

0.17

38

Threonine

49

-0.07

31

0.09

43

Serine

49

0.12

31

-0.14

43

0.06

31

-0.08

Asparagine

49

0.25’

31

-0.01

43

-0.25’

30

-0.04

-0.04

28

Glutamic Acid

48

0.05

30

0.01

42

Glutamine

49

0.05

31

0.07

43

-0.03

31

0.05 -0.15

0.26 -0.08

0.13

30

-0.01

31

-0.20

Proline

49

0.06

31

-0.17

41

Glycine

49

0.00

31

-0.07

43

0.361

31

Alanine

49

0.08

31

-0.08

43

0.01

31

-0.01

Citrulline

46

-0.341

31

-0.20

43

0.08

30

-0.35’

‘Y-Aminobutyric acid

48

0.06

31

-0.29

43

0.27

Valine

49

0.03

30

-0.09

43

Cystine

47

0.09

26

-0.2 1

Methionine

48

0.06

29

0.06

lsoleucine

49

0.00

31

Leucine

49

0.02

31

Tyrosine

49

0.04

31

Phenylalanine

49

0.10

31

Ornithine

49

31

Lysine

49

Histidine

49

Tryptophan

47

Arginine

47

I.Q. vs. age

49

-0.02 0.431

31

’

0.34’

31

0.06

0.08

31

-0.09

42

0.14

29

-0.22

42

0.17

30

-0.15

0.03

43

0.04

31

-0.08

43

0.05

31

-0.11

0.09

43

0.07

31

-0.19

0.09

43

0.24

31

-0.09

0.24

43

31

0.02 -0.08

-0.11

-0.01

0.02

43

0.08

31

31

0.40t

42

0.02

31

0.00

0.24’

31

0.41 t

43

0.04

31

0.02

0.13

31

0.08

43

0.01

31

0.02

43

0.03

31

0.06

-0.08

-0.07

31

-0.04

‘p 5 0.10. p 5 0.05. tp< 0.02.

t

All p values two-tailed

noted. Recently, Perry et al.14 revived the hypothesis that the mental and neurologic abnormalities characteristic of untreated patients with phenylketonuria might occur as a result of their lowered levels of plasma glutamine. Subsequent reports from other laboratories”-” failed to support this hypothesis, however. If lowered plasma glutamine levels were of paramount importance in the pathology of phenylketonuria, it might be expected that variations in glutamine levels of normal children might have some effect on their mental performance. The relationships between plasma glutamine and I.Q. observed here for normal children were near zero (O.Ol-0.07), despite the fact that the plasma glutamine range for these children (0.455 to 0.766 pmole/ml) extends from the low levels characteristic of the phenylketonuric subjects of Perry et al. to considerably greater than the mean value reported for his control group.

PLASMA

AMINO

ACIDS.

VIII.

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WITH

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1441

ACKNOWlEDGMENT We thank Linda Furness, Helen McCracken, and Patricia Block for assistance in performing amino acid analyses and Darryl Bloom, Pamela Hogarty, and Nancy Hurlburt for assembling I.Q. data and making statistical evaluations. REFERENCES 1. Waelsch H: Metabolism of glutamine, in Elliot KAC, Page IH, Quastel JH (eds): Neurochemistry. Springfield, Ill, Thomas, 1955, p 173 2. Roberts E, Eidelberg E: Metabolic and neurophysioiogical roles of y-aminobutyric acid. Int Rev Neurobiol2:279, 1960 3. Davison AN, Kaczmarek LK: Taurine-a possible neurotransmitter? Nature 234:107, 1971 4. Logan WJ, Snyder SH: Unique high affinity uptake system for glycine, glutamic and aspartic acids in central nervous tissue of the rat. Nature 234:297, 1971 5. Molinoff PB, Axelrod J: Biochemistry of catecholamines. Ann Rev Biochem 4Oz465, 1971 6. Gal EM, Poczick M, Marshall FD Jr: Hydroxylation of tryptophan to 5-hydroxytryptophan by brain tissue in vivo. Biochem Biophys Res Comm 12:39, 1963 7. Viteri F, Behlr M, Arroyave G: Clinical aspects of protein malnutrition, in Munro HN, Allison GB (eds): Mammalian Protein Metabolism, ~012. New York, Academic Press, 1964, p 523 8. Schwerin P, Bessman SP, Waelsch H: The uptake of glutamic acid and glutamine by brain and other tissues of the rat and mouse. J Biol Chem 184:37, 1950 9. Fernstrom JD, Wurtman RJ: Brain serotonin content: Physiological dependence on plasma tryptophan levels. Science 173: 149, 1971

10. Armstrong MD, Stave U: A study of plasma free amino acid levels. III. Variations during growth and aging. Metabolism, 22:57 I, I973 Il. Armstrong MD, Stave U: A study of plasma free amino acid levels. I. Study of factors affecting validity of amino acid analyses. Metabolism, 22:549, 1973 12. Stetten D, Hearon JZ: Intellectual level measured by Army classification battery and serum uric acid concentration, Science 129: 1737, 1959 13. Kasl SV, Brooks GW, Rodgers WL: Serum uric acid and cholesterol in achievement behavior and motivations I. The relationship to ability, grades, test performance and motivation. JAMA 213:1158, 1970 14. Perry TL, Hansen S, Tischler B, et al: Glutamine depletion in phenylketonuria. A possible cause of the mental defect. N Engl J Med 282:761, 1970 15. McKean DM, Peterson NA: Glutamine in the phenylketonuric central nervous system. N Engl J Med 283:1364, 1970 16. Wong PWK, Berman JL, Partington MW, et ai: Glutamine in PKU. N Engl J Med 285:580, 1971 17. Colombo JP: Plasma glutamine in a phenylketonuric family with normal and defective family members. Arch Dis Child 46: 720,1971