Adenyl cyclase

Int. J. Oral Surg. 1978: 7:43-51

Review Article

(Key words: hormones; adenyl cyclase; AMP, cyclic)

Adenyl cyclase DAVID COPPE AND MICHAEL L. STEER

Department of Surgery, Harvard Medical School and Beth Israel Hospital, Boston, Massachusetts, U.S.A.

Certain hormones regulate the activity of their target cells by stimulating adenyl eyclase, which is an enzyme located within the target cell's plasma membrane. Adenyl cyclase catalyzes the formation of cyclic AMP, which is released into the cell and modulates cell functions. In this communication the characteristics of adenyl cyclase are reviewed. The coupling between hormone receptors and this enzyme is discussed as is the ability of agents such as hormones, ATP, magnesium, calcium, guanine nttcIeatides ,-rod prostaglandins to alter cyclase activity. Several diseases that result from derangements of the adcnyl cyclase system are known and the molecular bases for these diseases are discussed in this review. hBSTRACT

- -

(Received ]or publication t3 April, accepted 15 May 1977)

The ability of circulating hormones to alter the activity of certain "target cells" has been recognized for many years, although an understanding of the mechanisms underlying this p h e n o m e n o n has only recently been obtained. Some hormones, such as tke adrenal steroids, estrogens and androgens, have been shown to enter the target cell, combine with specific cytoplasmic receptors, and then migrate to the nuclear area, where they modulate cellular activity25. Other hormones, such as the catecholamines and peptide hormones, apparently do not enter the target celt but exert their effect through an interaction with receptors located on the celt surface~,4t, Because of the wide distribution of these hormones in nature and the great variety of cell types affected by these hormones, an understanding of the mechanisms by which they can

alter cellular function without penetrating the target cell is of great importance. In 1958, SUTHERLAND and coworkers reported that a novel nucleotide, adenosine 3', 5' cyclic monophosphate (cyclic AMP) was formed when liver cells were exposed to catecholamines. Shortly thereafter they discovered that cyclic A M P was formed from ATP by an enzyme called adenyl cyclase which was located in the plasma membrane and which was stimulated by catecholamine hormones 51 (Fig. 1). It is now recognized that the effects of m a n y peptide hormones as well as the beta-adrenergic catecholamines result from the ability of these hormones to stimulate adenyl cyclase and cause an elevation i n the cellular level of cycIic AMP. In this selective review, information describing the characteristics of adenyl cyc-

44

COPPE AND STEER NH2

2

OI

OI

OI

N•N•jH

O--P-O-P-O-R-O-CH?_ j O - .

OH

N

] Adenyl Cyclose

[ --

'

OH

Adenosine 5' Triphosphote (ATP)

)

0

- --U 0

]

O--

i

O-

I

+ -O-P-O-P-OH II II 0 0

OH

Pyrophosphate

Adenosine 3'5' Cyclic

Monophospho'te (Cyclic AMP) Fig. 1. Formation of cyclic AMP from ATP. Reproduced from Sam~l~'J0 with permission of the publisher.

lase will be summarized. In the accompanying communication, the means by which cyclic A M P regulates cell function will be discussed. It is hoped that this review will provide a general background in this important area of molecular biology and facilitate the critical evaluation of new data. CYCLIC AMP - T H E SECOND MESSENGER A wide variety of hormones have been found to be capable of stimulating adenyl cyclase (Table 1). These hormones appear to bind to specific hormone receptors located on the external surface of the target cell (plasma membrane). SUTrmRLAND and coworkers proposed the "second messenger concept" to explain how these hormones, binding to cell surface receptors, might modulate intra-cellular activitynL According to this concept, the circulating hormones are thought of as "first messengers" since they convey information from endocrine glands to the target cell. Binding of "first messengers" to receptors results in stimulation of adenyl cyclase and elaboration of

cyclic A M P (Fig. 2). Since the catalytic area of adenyl cyclase is located on the cytoplasmic surface of the plasma membrane, the cyclic A M P which is formed from ATP is released into the interior of the ceil, where it acts as the "secol~d messenger". It is generally believed that all of the effects of cyclic A M P on cell function can be explained by the ability of this nucleotide to stimulate a group of cytoplasmic enzymes called cyclic A M P - d e p e n d e n t protein kinases. Once activated, these protein kinases modulate the activity of other cellular enzymes thus leading to an accel-

Table 1. Hormones which stimulate adenyI cyclase Epinephrine Norepinephrine Dopamine Glucagon Parathormone Histamine Triiodothyronine Prostaglandins Secretin

Hypothalmic Releasing Factors MSH ACTH TSH LH FSH Vasopressin Serotonin Thyrocalcitonin

ADENYL CYCLASE

ENDOCRINE

~,~~

45

~ Mg2+.ATP

INACTIVE PROTEIN KINASE

ACTIVE PROTEIN KtNASE

INACTIVE ENZYME

LA7 (ACTIVE) ENZYME -%

m~ar~ Fig. 2. The adenyl cyclase system. Reproduced from STEER4D with permission of the publisher.

eration or inhibition of cellular processes such as secretion, transport, synthesis, etc. 44 Eventually, the cyclic AMP is metabolized to 5'-adenosine monophosphate (Y-AMP) by another enzyme called cyclic nucleotide phosphodiesterase, and the hormone's signal is thus dissipated (Fig. 2). S P E C I F I C I T Y OF RESPONSE Although cyclic A M P is believed to act as the second messenger for many different hormones in a wide variety of cells, the response to each hormone is quite specific. The explanation for this specificity of response appears to involve several factors. First, only those cells possessing the proper h o r m o n e receptors are capable of being stimulated b y the circulating "first messenger", and the magnitude of that stimulation will depend on the circulating hormone concentration as well as the affinity of the receptors for the hormone. Secondly, the nature of the response will depend on the presence and type of cyclic AMP-dependent protein kinases in the target cell.

Thirdly, the eventual cellular response will depend on the nature of that cell's activities which are responsive to modulation by activated cyclic AMP-dependent protein kinases. Finally, there is now evidence that a variety of cellular factors may act to regulate the relative responsivity of adenyl cyclase to hormonal stimulation.

Regulation of adenyl eyclase Adenyl cyclase, as noted above, is an enzyme located within the plasma membrane. Like all enzymes, its activity is expressed as the rate at which the substrate (ATP) is converted into the product (cyclic A M P ) and this rate is, in part, determined by the availability (concentration) of substrate (ATP). The activity of many enzymes, particularly those acting at key points in metabolic pathways, is also modulated by other agents which are called "regulatory agents"20,36. In the case of adenyl cyclase, a variety of regulatory agents have been

46

COPPE AND STEER

identified. Each of these appears to have its own specific binding site, and occupancy of these '~ sites" either increases or decreases the catalytic activity of adenyl cyclase. F o r reasons of clarity, we will assume that all of the various regulatory sites are part of one macromolecular complex (Fig. 2). Thus, binding sites for the substrate (catalytic site), hormone (hormone receptor) and other regulators such as cations, nucleotides and prostaglandins can be considered. This is probably an oversimplification as some of these agents may bind to molecules other than adenyl cyclase itself. For example, there is now evidence that hormone receptors and adenyl cyclase are parts of separate molecules~,40 floating on opposite surfaces of the lipid core of the membrane and experiencing chance collisions with each otherlS. We will discuss the regulation of adenyl cyclase by 1) substrate (ATP), 2) hormones, 3) cations (Ca% Mg2+), 4) nucleotides (GTP), and 5) prostaglandins. It is likely that regulation of adenyl cyclase activity is only one of the ways in which these agents act in controlhng cell function. 1) S u b s t r a t e the catalytic s i t e - The activity of an enzyme is usually expressed as the rate at which substrate is converted into product. Among other factors, this rate of catalysis is determined by the concentration o f substrate available to the catalytic site of the enzyme. The concentration of substrate which allows half-maximal activity is a rough approximation of the binding constant (affinity) for substrate. I t is now generally believed that the substrate for adenyl cyclase is the complex of magnesium and A T P (Mg~+-H+-ATP) and other forms of A T P may act as competitive inhibitors 4~. The concentration of substrate giving half-maximal activity varies between i N 10-SM and 5 X 10-~M and the pH opt i m u m is near 7.44k F r o m these considerations it is clear that alterations in the eel-

lular concentrations of A T P and Mg e+ as well as pH may have profound effects on adenyl cyclase activity. 2) H o r m o n e s - hormone receptors - The physiologically relevant sites at which hormones bind to ceils are called hormone receptors. Receptors for hormones regulating adenyl cyclase are believed to face outward from the surface of the plasma membrane. Some of the hormones which have been found capable of stimulating adenyl cyclase are listed in Table 1. It appears that ~-adrenergic receptor binding, but not a-adrenergic receptor binding, leads to adenyl cyclase stimulation 44. The molecular structure of hormone receptors is not known, although they are believed to be at least in part proteins. Binding of hormones to receptors is dependent upon the compatibility of hormone and receptor structure so that each type of receptor can only accommodate a specific type of hormone. Minor modifications in hormone structure may either increase or decrease the "tightness" with which these hormones are bound to their receptors. Similarly, modifications in receptor structure may alter the receptor's affinity for certain hormones. This is most clearly seen in the case of catecholamine receptors, which have actually been divided into classes according to their relative affinity for catecholamines. Thus, the a-adrenergic receptors bind catecholamines in the following order of affinity: phenylephrine 2> epinephrine ---= norepinephrine 2> isoproterenol; whereas [3-adrenergic receptors bind catecholamines in the following order: isoproterenol 2> norepinephrine = epinephrine 2> phenylephrine 10. Only the l-stereoisomers of these hormones appear to bind1, t6. Furthermore, specificity for antagonists is also noted to depend on antagonist structure such that propranolol blocks only ~-receptors while phentolamine blocks only a-receptors tg.

ADENYL CYCLASE Little is k n o w n of the number and density of h o r m o n e receptors on most target celt surfaces. However, in one situation, the t u r k e y erythrocyte, each cell was f o u n d to possess 600-1,000 ~-adrenergic receptors~, ~-~ with an average receptor density of 1-2 per ~tmz. Whether or not these r e c e p t o r s are randomly distributed across t h e cell surface or exist in clusters is unknown. In some cases, however, cooperative interactions between receptors have b e e n noted 8a, suggesting that they may exist in clusters. Loss of receptors has been rep o r t e d to occur in some cell types following exposure to hormones '-'s, and it has been suggested that this phenomenon may exp l a i n desensitization. W e h a v e very little understanding of the events which couple hormone binding to adenyl cyclase activation. Data has accum u l a t e d which indicates that membrane phospholipids play a key role in this process30,s0,43 and, as we shall see later, there is also evidence suggesting that guanyl nucleotides participate in the coupling bet w e e n h o r m o n e receptor binding and aden y l cyclase activation. 3) C a t i o n s - The cations of magnesium a n d calcium have dramatic effects upon a d e n y l cyclase from many cell types. As a l r e a d y noted 45, Mg 2+ acts as part of the s u b s t r a t e (Mg~+-H+-ATP). This cation also a p p e a r s to enhance activity by another m e c h a n i s m which involves binding to specific Mg~+-binding sites on the enzyme4,50. M o s t adenyl cyclases are inhibited by Ca 2§ and, in one case, Ca~+-inhibition was shown to result from Ca 2§ binding to a cluster of calcium binding sites 50. Thus, alterations in cellular concentrations of Ca 2+ and Mg 2. m a y result in changes in adenyl cyclase activity in response to hormone stimulation. 4) N u c l e o t i d e s Most adenyl cyclases a p p e a r to require the presence of guanine nucleotides for activity in response to horm o n a l stimulation. I t is believed that GTP

47

is the physiologically important agent but most studies have utilized the GTP analogue 5'-guanylyMmidodiphosphate (Gpp (NH)p)~B,-%~,33,4'',45-4s. Most of the currently available evidence suggests that GTP directly activates adenyl cyclase by binding to a specific guanine nucleotide site and that hormones function by facilitating GTP bindingS1,4~. 5) P r o s t a g l a n d i n s - The prostaglandins are a group of 20-carbon fatty acids synthesized within many cell membranes that appear to be intrinsic regulators of adenyl cyclase for that particular cell. A variety of prostaglandins have now been identified and in some tissues the different prostaglandins may have differing effects on the adenyl eyclase. A s with the other regulators of adenyl cyclase activity, the prostaglandins probably act by binding to regulatory sites but the mechanism of their effect is not known.

Fluoride The fluoride ion has been found to stimulate adenyl cyclase activity in broken cell preparations from a wide variety of tissues4L F o r the most part, however, fluoride does not stimulate adenyl cyclase in intact cells. In broken cells, relatively high fluoride concentrations are required (1 • 10-2M) for stimulation. The mechanism of this "fluoride effect" is not known but it has been suggested that fluoride works by eliminating preexisting inhibitory forces which m a y be present 11. The fluoride effects on adenyl cyclase are probably of no physiological importance since it is only seen in broken cells.

Adenyl cyclase and disease The adenyl cyclase system consists of many components (Fig. 1): 1) hormone receptors binding specific hormones; 2) adenyl cyc-

48

COPPE AND STEER

lase catalytic unit catalyzing the formation of cyclic A M P ; 3) regulatory sites for cations and nucleotides altering the response of the catalytic unit to stimulation; 4) cyclic AMP-dependent protein kinases binding cyclic AMP and thereby becoming converted from inactive to active protein kinases; 5) intracellular enzymes, which are activated or inhibited when they are phosphorylated by the acti,~ated cyclic A M P dependent protein kinases; 6) phosphoprorein phosphatases, which dephosphorylate the intracellular enzyme systems that were phosphorylated by the cyclic AMP-dependent protein kinases; 7) cyclic nucleotide phosphodiesterase, which breaks down cyclic A M P by converting it to 5" adenosine monophosphate. A d e n y l cyclase-related diseases may represent changes in one or more components of this system. A t the present time, only a few diseases have been proven to be related to the adenyl eyclase system. CHOLERA The Vibrio c o m m a , which causes cholera, elaborates a toxin that binds to the plasma membrane of m a n y types of cells 12-14. Cholera toxin, by binding to intestinal cells, activates adenyl eyclase and causes an increase in intestinal cell cyclic A M P levels. This results in net fluid and electrolyte secretion rather than absorption by the intestine '-'4 and causes the profound diarrhea that is found in the disease cholera. N E P H R O G E N I C D I A B E T E S INSIPIDUS Antidiuretic hormone ( A D H ) stimulates the adenyl cyclase in cells of the distal collecting tubule of the kidney and elevated cyclic AMP levels cause an increased permeability of these ceils to water, which then passes out of the collecting tubules into the hypertonic renal medulla and leads to concentration of the urine under normal eonditionss,7,-~,38 There is a type of ne-

phrogenic diabetes insipidus, however, in which patients have normal or elevated circulating levels of A D H but the cells of the distal collecting tubule do n o t respondlS. It it not known, at present, which of the elements of the adenyl cyclase system is defective in this disease. PSEUDOHYPOPARATHYROIDISM Normally, p a r a t h o r m o n e stimulates cyclic A M P production in renal cortical ceils and osteoclasts in bone resulting in p h o s p h a t e excretion from the kidney and resorption and calcium liberation from bone 7-",3L I n pseudohypoparathyroidism, both of these responses are lacking. N o r m a l or e l e v a t e d parathormone levels are found in t h e circulating blood but the end organs (kidneys and bone) fail to respond 10. I t has been suggested that either the h o r m o n e r e c e p t o r s or the enzyme itself is defective in this disease. G L Y C O G E N S T O R A G E DISEASES The adenyl cyclase system plays a c e n t r a l role in regulating glycogen metabolism because glycogen breakdown (by the e n z y m e glycogen phosphorylase) is stimulated by cyclic AMP, and glycogen synthesis (by the enzyme glycogen synthetase) is inhibited by cyclic A M P 44, A t least two forms of glycogen storage disease have now been identified which appear to represent defects in the adenyl cyclase system because of the absence of specific key enzymes. M c A r d l e ' s disease results from an absence of muscIc glycogen phosphorylase17,8~, and H e r s ' disease is caused by a deficiency of liver glycogen phosphorylasee-% ~. SUSPECTED A D E N Y L C Y C L A S E DISEASES I t is now recognized that the adenyl cyclase system is an extremely important e l e m e n t in a large number of cells. In essence, it represents the receiver and amplifier f o r

ADENYL CYCLASE effeeting the cellular response to many hormones. It will, therefore, not be surprising if defects of this system are found to be the basis for many c o m m o n diseases. Indeed, there is evidence to suggest t h a t some forms of hypertension, diabetes and obesity m a y result from defects in the adenyl cycIase system. F a r t h e r studies, however, will be necessary before these diseases can be proven to result from defects in the adenyl cyclase system,

References 1. ATLAS, D., STEER, M. L. & LEVITZKI, A.: Stereospecific binding of propranolol and catecholamines to the ~-adrenergic receptor. Proc. Nat. Acad. Sci. 1974: 71: 42464248. 2. BILEZI:KIAN, J. P. & AUR.Bt~CH, O. D.: The effects of nucleotides on the expression of ~-adrenergic adenylate cyclase activity in membranes from turkey erythrocytes. J. Biol. Chem. 1974: 249: 157-161. 3. BmNP,AUrCmR, L.: Hormone sensitive adenylyl cyclases. Useful models for studying hormone receptor functions in cell free systems. Biochim. Biophys. Acta 1973: 300: 129. 4. BIItNBAUMER, L., POHL, S. L. & RODBBLL, M.: Adenyl cyclase in fat cells. I. Properties and the effects of adrenocorticotropin and fluoride. J. Biol. Chem. 1969: 244: 3468-3476. 5. BROWN, E., CLARKE, D. L., Roux, V. & SHERMAN, G. H.: The stimulation of adenosine 3',5'-monophosphate by antidiuretic factors. J. Biol. Chem. 1963: 238: 852. 6. CARON, M. G. & LEFKOWITZ, R. J.: ~adrenergic receptors: Solubilization of (-) [aH] alprenolol binding sites from frog erythrocyte membranes. Biochem. Biophys. Res. Comm. 1976: 68: 315-322. 7. C-MaSE, L. R. & AURBACH, G. D.: Renal adenyl cyclase: Anatomically separate sites for parathyroid hormone and vasopressin. Science 1968: 159: 545. 8. CrusE, L. R. & AURBACH, G. D.: The effect of parathyroid hormone on the concentration of adenosine 3', 5'-monophosphate in skeletal tissue in vitro. J. Biol. Chem. 1970: 245: 1520-1526. 9. CHASe, L. R., FEDA~:, S. A. & AURI~^CH,

49

G. D.: Activation of skeletai adenyl cyclase by parathyroid hormone in vitro. Endocrinology 1969: 84: 761-768. 10. CRUSE, L. R,, NELSON, G. L. & Ar.m~ACH, G. D.: Pseudohypoparathyroidism. Defeclive excretion of 3', 5'-AMP in response to parathyroid hormone. J. Clin. Invest. 1969: 48: 1832-1844. 11. CONSTAbrroPotmOS, A. & NAJJAR, V.: The activation of adenylate cyclase. II. The postulated presence of (a) adenylate cyclase in a phospho (inhibited) form (b) a dephospho (activated) form with a cyclic adenylate stimulated membrane protein kinase. Biochem. Biophys. Res. Comm. 1973: 53: 794. 12. C'UAa'~mC~SAS, P.: Cholera toxin-fat cell interaction and the mechanism of action of the lipolytic response. Biochemistry 1973: 12: 3567-3577. 13. Cr.rATIteCAShS, P.: Interaction of Vibrio cholerae enterotoxin with cell membranes. Biochemistry 1973: 12: 3547-3558. 14. CtrA'n~CASAS, P.: Vibrio cholerae choleragenoid mechanism of inhibition of cholera toxin action. Biochemistry t973: 12: 35773581. 15. C'oKrm~cASAS, P.: Membrane receptors. Ann. Rev. Biochem. 1974: 43: 269-214. 16. CUATRIECA~S,P., TELL, G. R.. E., SICA, V., PaXtKH, I. & CHANt, K. I.: Noradrenaline binding and the search for catecholamine receptors. Nature 1973: 247: 92-97. 17. D.~WSON, D. M., SPONO, F. L. & ~ I N O TON, J. F.: McArdle's Disease: Lack of muscle phosphorylase. Ann. Intern. Med. I968: 69: 229-235. 28. FrCHM~Uq,M. P. & BRoozcea, G.: Deficient renal cyclic 3', 5' monophosphate production in nephrogenic diabetes insipidus. 1. Clin. Endocrinol. Metab. 1972: 35: 35-47. 19. FtmCHOOTT, R. F.: The classification of adrenoreceptors (adrenergic receptors). An evaluation from the standpoint of receptor theory. In: Handbook o/ experimental pharmacology, Vol. 33. Springer-Verlag, Berlin - Heidelberg - New York 1972, p. 283. 20. GEru3ART, J. C. & PaRDEE, A. B.: The enzymology of control by feedback inhibilion. J. Biol. Chem. 1962: 237: 891. 21. GRANTa-ta,M, J. J. & BURG, M. B.: Effect of vasopressin and cyclic AMP on permeability of isolated collecting tubules. Atn. ]. Physiol. 1966: 211: 255-259. 22. H~as, H. G.: Etudes enzymotiques sur

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COPPE AND STEER

fragments hepatiques. Rev. Int. Hepat. 1959: 9: 35. 23. HuG, G. R. & SCHOnERT, W. K.: Type VI glycogenosis: Biochemical demonstration of liver phosphorylase deficiency. Biochem. Biophys. Res. Comm. 1970: 41: 1178. 24. KrMBEaQ, D. V., FIELD, M., JOHNSON, J., HENDERSON, A. & GERSHON, E.: Stimulation of intestinal mucosal adenyl cyclase by cholera enterotoxin and prostaglandins. J. Clin. Invest. 1971: 50: 1218-1230. 25. KING, R, J. B. & MAINWARING, W. L P.: Steroid-cell interactions. University Park Press, Baltimore 1974. 26. KRISNNA, G., HARWOOD, J. P., BARBER, A. J. & JAMmSON, A. J.: Requirement for guanosine triphosphate in the prostaglandin activrLtion of adenylate cyclase of platelet membranes. J. Biol. Chem. 1972: 247: 2253-2254. 27. LEFKOWITZ, R. J.: Stimulation of catecholamine-sensitive adenylate cyclase by 5'-guanylyl-imidodiphosphat e. J. Biol. Chem. 1974: 249: 6119-6124. 28. LEI~KO~rrz, R. J.: The (5-adrenergic receptor. Li/e Sci. 1976: 18: 461--472. 29. LERAu F., CHAMnAtYr, A. M. & HANOUNE, J.: Role of GTP in epinephrine and glucagon activation of adenyl cyclase of liver plasma membrane. Biochem. Biophys. Res. Comm. 1972: 48: 1385. 30. I 2 w u G. S.: Restoration of norepinephrine responsiveness of solubilized myocardial adenylate cyclase by phosphatidylinositol. J. Biol. Chem. 1971: 246: 74057407. 31. LaswTzla, A.: The role of GTP in the activation of adenylate cyclase. Biochem. Biophys. Res. Comm. 1977: 74: 1154-1159. 32. LEv~rzKI, A., ATLAS, D. & STEER, M. L.: The binding characteristics and number of [:3-adrenergic receptors on the turkey erythrocyte. Proc. Nat. Acad. Sci. 1974: 71: 2773-2776. 33. L~W, G. S.: Restoration of glucagon responsiveness of solubilized myocardial adenyl cyclase by phosphatidylserine. Biochem. Biophys. Res. Comm. 1971: 43: i08. 34. LI~BmD, L. E., D~MEYTS, P. & LEFKOWXTZ, R. L: fS-adrenergie receptors: Evidence for negative cooperativity. Biochem. Biophys. Res. Comm. 1975: 64: 1160-1168. 35. McAItDLE, B.: Myopathy due to a defect in muscle glycogen breakdown. Clin. Scl. 1951: 10: 13.

36. MONOD, J., CI-IANGEUX,J. P. & JACOB, F.: Allosteric proteins and cellular control systems. J. Mol. Biol. 1963: 6: 306. 37. MURAD, F., BREWER, H. B. & VAUGtL4.N, M.: Effect of thyrocalcitonin on adenosine 3':5'-cyclic phosphate formation by rat kidney and bone. Proc. Nat. Acad. Sci. 1970: 65: 446-453. 38. OgLOFF, J. & HANDLER, J. S.: The similarity of effects of vasopressin, adenosine 3'5'-phosphate (cyclic 3',5' AMP) and theophylline on the toad bladder. J. Clin. Invest. 1962: 41: 702. 39. ORLY, J. & SCI-I~,MM, M.: Fatty acids as modulators of membrane functions: Catecholamine-activated adenylate cyclase of the turkey erythrocyte. Proc. Nat. Acad. Sci. 1975: 72: 3433-3437. 40. OKLY, J. • SCHRAMM,M.: Coupling of the catecholamine receptor from one cell with an adenylate eyclase from another cell by cell fusion. Proc. Nat. Acad. Sci. 1977: in press. 41. PEaKINS, J. P.: Adenyl cyclase. In: GI~Em,~GAnD, P. & ROmSON, G. A. (eds.): A d vances in cyclic nucleotide research, Vol. 3. Raven Press, New York 1973, pp. 1-64. 42. PI~EUFFER,T. & I"~ELMREICH,E. J. M.: Activation of pigeon erythrocyte membrane adenylate cyclase by guanylnucleotide analogues and separation of a nucleotide binding protein. J. Biol. Chem. 1975: 250: 867876, 43. POHL, S. L., KRANS, M. J., KOZY~FF, V., BI3RNBALrMER,L. & RODBELL, M.: The glucagon-sensitive adenyl eyclase system in plasma membranes of rat liver. J. Biol. Chem. 1971: 246: 4447-4454. 44. ROmSON, G. A., BUTCHER, R. W. & SUTHERLAND, E. W.: Cyclic A M P . Academic Press, New York 1971. 45. RODBELL, M., LIN, M. C., SALOmON, Y,, LONDOS, C., HARWOOD, J. P., MARTIN, B. R., RENDELL, M. & BEn.MAN, M,: The role of adenine and guanine nucleotides in the activity and response of adenylate cyclase systems to hormones. Evidence for multi-site transitions. In: GR~.ENGARD, P. & ROBISON, G. A. (eds.): Advances in cyclic nucleotide research, Vol. 5. Raven Press, New York 1975, pp. 3-29. 46. SCHRAMM, M.: The catecholamine-responsive adenylate cyclase system and its modification by 5' guanylylimidodiphosphate. In: GREm,roARD, P. & ROBISON, G'. A. (eds.): Advances in cyclic nucleotide re-

ADENYL CYCLASE search, Vol. 5. Raven Press, New York 1975, pp. 105-115. 47. S~VmLA, N., STEER, M. L. & LEVTTZFd,A.: Synergistic activation of adenylate cyclase by guanylyl imidodiphosphate and epinephrine. BiochemistJ3~ 1976: 15: 3493-3499. 48. SPmGEL, A. M., BROWN, E. M., FEDAK, S. A., WOODARD, C. J. & AUR~ACa, G. D.: Holocatalytic state of adenylate cyctase in turkey erythrocyte membranes: Formation with guanylylimidodiphosphate plus isoproterenol without effect on affinity of [3receptor. J. Cyclic Nucleotide Res. 1976: 2: 47-56.

Address: Ivlichael L. Steer, M.D. Beth lsrael Hospital 330 Brookline Avenue Boston, M A 02215 U.S.A.

51

49. STEER,~r L.: AdenyI cyclase. Ann. Surg. 1975: 182: 603-609. 50. STEER, M, L, & LEVITZ~, A.: The control of adenylate cyclase by calcium in turkey erythroeyte ghosts. J. Biol. Chem. 1975: 250: 2080-2084. 51. SUTHERLAND, E. W., RXLL, T. W. & MENON, T.: Adenyl cyclase: L Distribution, preparation and properties. J. Biol. Chem. 1962: 237: 1220. 52. StVFH~RLAND, E. W., Ro~Isot,~, G. A. & BUTC~R, R. W.: Some aspects of the biological role of adenosine 3', 5'-monophosphate (cyclic AMP). Circulation 1968: 37: 279-306,