Separation and investigation of the regulatory properties of two forms of cyclic nucleotide phosphodiesterase from rabbit heart--sensitive and insensitive to Ca-dependent regulator protein]

Two forms of cyclic nucleotide phosphodiesterase (ES 3.1.4.17)--PDE-I and PDE-II--sensitive and resistant to Ca-dependent protein regulator, were isolated from the soluble fraction of rabbit heart by chromatography on DEAE-cellulose. Both forms of enzyme are inhibited by 30--50% by Ca2+ (10(-4) M). Addition of Ca-dependent protein regulator activates PDE-I and eliminates Ca2+-induced inhibition of PDE-II. In heart extract Ca2+ increases the phosphodiesterase activity 1.5-fold. The amount of PDE-I makes up to about 10% of total phosphodiesterase activity of the heart; that of PDE-II is about 90%. In the presence of Ca-dependent protein regulator the rate of 3', 5'-AMP hydrolysis by PDE-I is increased 5--15-fold, while that of 3', 5'-GMP hydrolysis only 2.5-fold. Both PDE-I and PDE-II have close Km values for substrates--(3.5--4.0).10(-6) M for 3', 5'-AMP and 14.10(-6) M for 3', 5'-GMP. Inhibition by Ca2+ and effect of Ca-dependent protein regulator manifest themselves in changes in V for cyclic nucleotide hydrolysis and do not alter the Km value for the enzyme.     

Ca2+-calmodulin-activated cyclic nucleotide phosphodiesterase from the rabbit myometrium

Two forms of soluble phosphodiesterase of cyclic nucleotides separating by DEAE-cellulose ion-exchange chromatography and not only differing in physicochemical and catalytic parameters but also differently regulated by calmodulin are found in the doe myometrium. Calmodulin with 10(-7)-10(-5) M concentrations of Ca2+ promotes the two-fold activation of the 3':5'-AMP (but not of 3':5'-GMP) hydrolysis by the first form of phosphodiesterase. Trifluoperazine (10 microM) lowers the activating action of calmodulin. The second form of soluble phosphodiesterase is not sensitive to the action of both calmodulin and Ca2+. 3':5'-GMP (10 microM) inhibits the 3':5'-AMP hydrolysis by the first form of phosphodiesterase; calmodulin exerts no effect on this process. The data obtained testify to the possible participation of Ca2+ and calmodulin in Ca2+-calmodulin-dependent phosphodiesterase regulation of the content of cyclic nucleotides (3':5'-AMP, in particular) in the doe myometrium.     

Properties of cyclic nucleotide phosphodiesterase of the rabbit myometrium in various functional states

Chromatography on DEAE-cellulose of a soluble sulfate-precipitated fraction of cyclic nucleotide phosphodiesterase from rabbit myometrium revealed two 3':5'-GMP and 3':5'-AMP-hydrolase activities. 3':5'-GMP phosphodiesterase (fraction I) was eluted with 0.15-0.23 M NaCl, while 3':5'-AMP phosphodiesterase (fraction II) with 0.2-0.35 M NaCl. 3':5'-GMP phosphodiesterase hydrolyzed 3':5'-GMP with Km = 14 microM and V = 5.25 nmol . min . mg of protein, while 3':5'-AMP phosphodiesterase hydrolyzed both cyclic nucleotides with Km for 3':5'-GMP equal to 12 microM and V = 1.33 nmol . min . mg of protein; the Km value for 3':5'-AMP was 3.6 and 30.5 microM, respectively; the corresponding values of V were 0.28 and 0.97 nmol . min . mg of protein. In late pregnancy, the level of the 3':5'-AMP hydrolase activity of rabbit myometrium was significantly elevated in parallel with an increase in V, predominantly for the enzyme with a low affinity for 3':5'-AMP. The 3':5'-GMP hydrolase activity and V were largely decreased for both phosphodiesterase fractions; the Km value for fraction I was also diminished. During labour, the rate of 3':5'-AMP hydrolysis by myometrium phosphodiesterase was decreased down to the level typical of functional rest. The rate of 3':5'-GMP hydrolysis during the same period by fraction I remained at a low level, i. e., as in pregnancy, while that of fraction II was increased up to the level typical of functional rest.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cyclic nucleotide phosphodiesterase from wheat sprouts. Purification, properties, effect of systemic fungicides

Cyclic nucleotide phosphodiesterase from wheat sprouts was isolated and partially purified. The molecular weight of the enzyme is about 83 000. The enzyme activity sharply rises as the inhibiting factors present in the homogenate are separated. The pH optimum of the enzymatic reaction is 4,8. Divalent cations (Mg2+, Mn2+, Cu2+) within the concentration range of 1--5 mM and complexons (EDTA, EGTA) at the concentration of 1 mM do not affect the PDE activity. The temperature optimum for the reaction is 60 degrees. The enzyme hydrolyzes 3' : 5'-AMP, 3' : 5'-GMP and 2':3'-AMP. The Km value for cAMP is 4 . 10(-3) M. The enzyme activity is inhibited by chemical agents possessing the fungicide activity, the strongest effect being exerted by anylate.     

Effect of training and physical load on 3':5'-AMP content and activity fo the enzymes of its metabolism in rat muscle

It is established tha in skeletal muscles of dormant animals under the influence of training the 3':5'-AMP content and the activity of adenylate cyclase increase; that of phosphodiesterase remains unchanged. A long physical load causes no changes in the 3':5"-AMP content in the skeletal muscles of the trained rats as compared to its content in the intact animals but evokes a decrease as compared to its level in the trained rats muscles. The activity of adenylate cyclase in the skeletal muscles of the trained rats under long physical load lowers, that of 3':5'-AMP phosphodiesterase also decreases but to a less extent. The found changes in the 3':5'-AMP metabolism in the trained animals skeletal muscles evidence for a possible participation of the 3':5'-AMP system in development of the organism adaptation to higher physical loads.     

Regulation of urocaninase activity in the liver: role of 3',5'-AMP]

A dependence of rat liver urocaninase activity on the agents affecting the adenylate cyclase system was studied in vitro and in vivo. Urocaninase is considerably activated after the injection of glucagone, NaF, theophylline and 3',5'-AMP. Under conditions optimal for the protein kinase activity of phosphorylase the urocaninase of liver extracts was activated 7-fold on the average. The nezyme retains its activity after gel-filtration through Sephadex G-25 and is capable of inactivation in the presence of Mg2+ and of reactivation after addition of ATP and 3',5'-AMP. These data suggest a possibility of regulation of mammalian liver urocaninase activity by 3',5'-AMP-dependent phosphorylation of the enzyme. Derivatives of hypoxanthine (theophylline and caffeine) in concentration 10(-4) M activate urocaninase in liver extracts 2--3 and 1.5-fold respectively. The activation is probably not due to the 3',5'-AMP phosphodiesterase inhibition, since another phosphodiesterase inhibitor--papaverine--has no activating effect on urocaninase.     

Isolation and properties of 2 forms of cyclic nucleotide phosphodiesterase from the rat brain under normal conditions and after irradiation

It is established that the functional activity of two phosphodiesterase forms--phosphodiesterase I (Ca2+-calmodulin-sensitive) and phosphodiesterase II (Ca2+-calmodulin-insensitive), isolated from grey matter of the irradiated rat brain varies essentially in comparison with that of the normal rats. In the early period of acute radiation injury both phosphodiesterase I sensitivity to calmodulin and phosphodiesterase II special activity under hydrolysis of 3', 5'-GMP decrease but phosphodiesterase I special activity under hydrolysis of 3', 5'-GMP increases. The investigation of temperature dependence of phosphodiesterase I and phosphodiesterase II activations revealed changes in character of curves, the temperature optimum under irradiation being unchanged and inflections appearing on the Arrhenius curves.     

Sequence homology of the Ca2+-dependent regulator of cyclic nucleotide phosphodiesterase from rat testis with other Ca2+-binding proteins.

A Ca2+-dependent regulator protein of cyclic 3':5'-nucleotide phosphodiesterase (EC 3.1.4.17) has previously been isolated from rat testis and shown to be a heat-stable, Ca2+-binding protein with a molecular weight of approximately 17,000. The Ca2+-dependent regulator protein is also structurally similar to troponin-C, the Ca2+-binding component of muscle troponin and Ca2+ mediator of muscle contraction. The present report describes a partial amino acid sequence of the Ca2+-dependent regulator. The protein (148 amino acids) is 50% homologous with skeletal muscle troponin-C, but is 11 residues shorter than the muscle protein. The Ca2+-dependent regulator protein has an NH2-terminal sequence of acetyl-Ala-Asp-Glu, a COOH-terminal sequence of Thr-Ala-Lys and 1 residue of epsilon-trimethyllysine located at position 115. All of these properties are distinct from those of other homologous Ca2+-binding proteins. These properties may account for the biological specificities demonstrated by these proteins as compared to the Ca2+-dependent regulator protein. Based on the sequence and a comparison of the Ca2+-dependent regulator protein to other calcium-binding proteins, our data support the view that all of these moecules contain common sequences, especially at their proposed metal-binding sites.     

Repressible extracellular phosphodiesterases showing cyclic 2'',3''- and cyclic 3'',5''-nucleotide phosphodiesterase activities in Neurospora crassa.

Two molecular species of repressible extracellular phosphodiesterases showing cyclic 2',3'- and cyclic 3',5'-nucleotide phosphodiesterase activities were detected in mycelial culture media of wild-type Neurospora crassa and purified. The two molecular species were found to be monomeric and polymeric forms of an enzyme constituted of identical subunits having molecular weights of 50,000. This enzyme had the same electrophoretic mobility as repressible acid phosphatase. The enzyme designated repressible cyclic phosphodiesterase showed pH optima of 3.2 to 4.0 with a cyclic 3',5'-AMP substrate and 5.0 to 5.6 with a cyclic 2',3'-AMP substrate. Repressible cyclic phosphodiesterase was activated by MnCl2 and CoCl2 with cyclic 2',3'-AMP as substrate and was slightly activated by MnCl2 with cyclic 3',5'-AMP. The enzyme hydrolyzed cyclic 3',5'- and cyclic 2',3'-nucleotides, in addition to bis-rho-nitrophenyl phosphate, but not certain 5' -and 3'-nucleotides. 3'-GMP and 3'-CMP were hydrolyzed less efficiently. Mutant strains A1 (nuc-1) and B1 (nuc-2), which cannot utilize RNA or DNA as a sole source of phosphorus, were unable to produce repressible cyclic phosphodiesterase. The wild type (74A) and a heterocaryon between strains A1 and B1 produced the enzyme and showed growth on orthophosphate-free media containing cyclic 2',3'-AMP or cyclic 3',5'-AMP, whereas both mutants showed little or no growth on these media.     

Ca2+-dependent regulation of cyclic-AMP phosphodiesterase by parvalbumin.

Carp parvalbumin has been shown to activate rat brain phosphodiesterase in a Ca2+-dependent manner. The concentration of Ca2+ required for half-maximal stimulation is 1.4 X 10(-7) M, whereas rat testis Ca2+-dependent regulator (CDR) of phosphodiesterase required 1.2 X 10(-6) M Ca2+. The difference in the slopes of the two curves demonstrated that the activation induced by parvalbumin was not the result of a small contamination by CDR. In addition, it has been shown that Ca2+ binding to parvalbumin parallels its activation of phosphodiesterase. These data suggest that Ca2+ must bind to a single specific metal binding site before phosphodiesterase can be fully activated.     

The stimulating effect of cyclic AMP, glucagon and insulin on guanidine acetate-N-methyltransferase activity in rat liver and pancreas]

N6,O2'-dibutyrylcyclo-3',5'-AMP injected to intact rats alone or in combination with theophylline increases the activity of guanidine acetate methyltransferase (GAMT) in liver and pancreas. Cyclic 3',5'-AMP and its dibutyryl analog administered immediately or two hours after the suturing of common bile duct (SCBD) stimulate the increase of pancreatic GAMT activity 2-3 fold. Glucagon, injected intraabdominally simultaneously with SCBD and administration of theophylline, dramatically increases the theophylline effect on the GAMT activity. The freezing of rat pancreas pretreated witn secretin, a hormone structurally similar to glucagon, results in a 1.5-2-fold increase of creatine synthesis from S-adenosylmethionine and guanidinacetic acid. An hour after glucagon administration to intact rats the GAMT activity of liver increases 9 times. The effect of glucagon is enhanced by insulin. Cycloheximide inhibits the increase of GAMT activity, induced by glucagon or a combination of glucagon and insulin. Experiments on tissue homogenates demonstrate that 3',5'-AMP in concentrations of 10(-8) --10(-2) M does not affect the GAMT activity or to some extent inhibits the enzyme. The homogenate incubation in a medium containing 10(-5) M epinephrine or 10(-7) M caffeine and 5 mM Mg2+ leads to an increase in the GAMT activity. Oligomycin removes the stimulating effects of caffeine and Mg2+ on the enzyme activation. This is probably due to the presence of 3',5'-AMP-dependent protein kinase in the mechanism of GAMT activation by cyclic AMP.     

rac-Glycerol 1:2-cyclic phosphate 2-phosphodiesterase, a new soluble phosphodiesterase of mammalian tissues.

1. A soluble phosphodiesterase is present in mammalian tissues which rapidly hydrolyses enantiomorphs of rac-glycerol 1:2-cyclic phosphate, producing rac-glycerol 1-phosphate. 2. The enzyme has been purified up to 1700-fold by a combination of acetone precipitation and chromatography on DEAE-Sephadex A-50, Sephadex G-150 and hydroxyapatite. 3. The Km with glycerol cyclic phosphate as substrate is 7.2 mM, and the pH optimum broad (6.9--7.5). The molecular weight (by gel filtration) of the enzyme is approx. 35500. 4. The phosphodiesterase has no requirement for Ca2+ or Mg2+, but is stimulated by reducing agents (cysteine, dithiothreitol) and Fe2+. 5. The purified phosphodiesterase preparation also hydrolysed 3':5'-cyclic AMP, producing 5'-AMP exclusively, and 2':3'-cyclic AMP, forming 3'-AMP and 2'-AMP in the ratio 7:3. Bis-(p-nitrophenyl) phosphate was slowly hydrolysed, but other phosphodiesters tested were not attacked. 6. The phosphodiesterase is inhibited by theophylline and o-phenanthroline. It is inhibited by Pi and by a variety of phosphomonoesters, of which certain aromatic primary phosphates are particularly effective.     

Phosphotyrosine phosphatase: a novel phosphatase specific for phosphotyrosine, 2'-AMP and p-nitrophenylphosphate in rat brain

A unique phosphatase that selectively hydrolyzed phosphotyrosine and 2'-AMP at alkaline pH and p-nitrophenylphosphate at neutral pH was isolated from a cytosolic fraction of rat brain. The purified enzyme appeared homogenous on SDS-polyacrylamide gel electrophoresis and its molecular weight was estimated to be 42,000. The molecular weight of the native enzyme was 45,000 as determined by molecular sieve chromatography. These findings indicate that the native enzyme is a monomer protein. At pH 8.6, the enzyme hydrolyzed L-phosphotyrosine, D-phosphotyrosine, 2'-AMP, p-nitrophenylphosphate, 3'-AMP, 2'-GMP, and 3'-GMP; the ratio of its activities with these substrates was 100:96:115:68:39:25:16. Its Km values for L-phosphotyrosine, 2'-AMP, and p-nitrophenylphosphate were 0.8 X 10(-4) M, 1.4 X 10(-4) M, and 1.7 X 10(-4) M, respectively. At pH 7.4, the enzyme hydrolyzed p-nitrophenylphosphate, L-phosphotyrosine, and D-phosphotyrosine; the ratio of its activities with these compounds was 100:17:17, and its Km values for L-phosphotyrosine and p-nitrophenylphosphate were 1.8 X 10(-4) M and 2.0 X 10(-4) M, respectively. The enzyme activity was dependent on Mn2+ or Mg2+, and was strongly inhibited by 5'-nucleotides, pyrophosphate, and Zn2+. The enzyme was not sensitive to inhibitors of some well-characterized phosphatases such as NaF, molybdate, L(+)tartrate, tetramisole, vanadate, and lithium salt. The physiological role of the enzyme is discussed with respect to its activities toward phosphotyrosine, 2'-AMP, and p-nitrophenylphosphate.     

Kinetics and regulatory properties of the disulfide reductase enzyme from mouse liver]

Disulfide reductase (DSR) of mice liver supernatant is kinetically demonstrated as associating-dissociating oligomeric protein with positive homotropic cooperativity for the substrate. Cyclic 3',5'-AMP (10(-11)--10(-5) M) activates DSR and increases V, but does not change either [S]0,5, nor nH and does not shift the plot of specific activity versus the enzyme concentration. ATP, GTP, UTP, CTP, protamine, histone, Mg2+, Ca2+, EDTA (but not adenosine, 5'-AMP, 2'3'-AMP, ADP beef serum albumin) activated DSR. The effects of different modifiers are not summed up. Preincubation is essential for the action of the majority of the activators. Heating for 8 minutes at 55 degrees C desensitized completely DSR to all the modifiers without changing its catalytic activity, [S]0,5 and nH values. Possible mechanisms of activation of DSR, especially the involvement of protein kinase, are discussed.     

Cyclic 3',5'-AMP phosphodiesterase in Physarum polycephalum. I. Chemotaxis toward inhibitors and cyclic nucleotides.

The effect of several inhibitors of the enzyme cyclic 3',5'-AMP phosphodiesterase as chemoattractants in Physarum polycephalum was examined. Of the compounds tested, 4-(3-butoxy-4-methoxybenzyl)-2-imidazolidinone (Roche 20-1724/001) and 1-ethyl-4-(isopropylidinehydrazino)-1H-pyrazolo-(3,4-b)-pyridine-5-carboxylic acid ethyl ester, hydrochloride (Squibb 20009) were the most potent attractants. 3-Isobutyl-1-methyl xanthine, theophylline, and morin (a flavanoid) were moderate attractants and sometimes gave negative chemotaxis at high concentrations. Cyclic 3',5'-AMP was an effective, but not potent attractant. A repellent effect following the positive chemotactic action was sometimes observed with cyclic 3',5'-AMP at concentrations as high as 1 . 10(-2) M. Dibutyryl cyclic AMP appeared to be a somewhat more potent attractant than cyclic 3',5'-AMP. The 8-thiomethyl and 8-bromoderivatives of cyclic AMP, which are poorly hydrolyzed by the phosphodiesterase, were not attractants in Physarum. Possible participation of cyclic 3',5'-AMP in the directional movement in P. polycephalum is discussed.     

Identification of an activator protein for myosin light chain kinase as the Ca2+-dependent modulator protein

Myosin light chain kinase which phosphorylates g2 light chain of skeletal muscle myosin requires an activator for the activity (Yazawa, M., and Yagi, K (1977) J. Biochem. (Tokyo) 82, 287-289). This activator has now been identified as the modulator protein known to be a Ca2+-dependent regulator for phosphodiesterase, adenylate cyclase, and ATPases. The identification is based on the quantitative cross-reactivity of muscle activator protein and brain modulator protein in activating myosin light chain kinase and brain phosphodiesterase and identical properties of both proteins in regard to sensitivities to Ca2+, UV absorption spectra, UV absorption difference spectra with or without Ca2+, and mobilities upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis. In the presence of modulator protein, the activity of myosin light chain kinase was reversibly controlled by the physiological concentration of Ca2+. We suggest that two Ca2+-receptive proteins, i.e. modulator protein and troponin-C, may play roles in the contraction-relaxation cycle of skeletal muscle.     

Modification of calcium fluxes by dimethyl sulfoxide and 2-butoxyethanol in sarcoplasmic reticulum vesicles: a possible mechanism for skeletal muscle relaxation induced by dimethyl sulfoxide

In order to investigate the mechanism of skeletal muscle relaxation induced by dimethyl sulfoxide, 2-butoxyethanol and dimethyl sulfoxide were examined for their effects on 1) Ca2+ uptake into and efflux from sarcoplasmic reticulum vesicles prepared from rabbit fast skeletal muscle and crayfish tail muscle by the murexide method, 2) ATPase activities of rabbit reticulum vesicles, 3) the isolated phrenic nerve-diaphragm preparation of the rat and 4) crayfish opener muscle preparation. Ca2+ efflux rate from rabbit reticulum vesicles was markedly decreased with increasing concentrations (5-20% v/v) of dimethyl sulfoxide without affecting the maximum Ca2+ uptake by the reticulum. 2-Butoxyethanol showed quite contrary effects. Dimethyl sulfoxide strongly inhibited the activity of basal ATPase rather than of Ca2+-dependent ATPase. 2-Butoxyethanol did not significantly inhibit the activity of basal ATPase, but markedly increased Ca2+-dependent ATPase activity. Antagonisms between dimethyl sulfoxide and caffeine were demonstrated either in contractions of crayfish opener muscles or in the Ca2+ release from crayfish sarcoplasmic reticulum vesicles. These results indicate a possibility that dimethyl sulfoxide reversibly induces skeletal muscle relaxation mainly in the sarcoplasmic reticulum by means of decreasing the rate and the amount of Ca2+ release from the reticulum.     

Biological cross-reactivity of rat testis phosphodiesterase activator protein and rabbit skeletal muscle troponin-C.

Phosphodiesterase activator protein and troponin-C have been purified from rat testis and rabbit skeletal muscle, respectively. The two proteins appear to be structurally distinct since the activator protein migrates faster than troponin-C on sodium dodecyl sulfate-polyacrylamide gels. Each of the calcium-binding proteins will, however, substitute for the other in their respective biological systems. Testis activator protein forms a complex with rabbit muscle troponin subunits TnI and TnT soluble in low salt. This hybrid complex (AIT) can regulate rabbit skeletal muscle actomyosin ATPase activity. AIT regulation, although influenced by free Aa2+ levels, is distinct from that of native troponin. Likewise, muscle troponin-C can substitute for activator protein in the stimulation of cyclic nucleotide phosphodiesterase. Troponin-C will fully stimulate phosphodiesterase although its affinity is 600-fold lower than that of activator protein. Ca2+ regulation studies demonstrate that both proteins require micormolar levels of free Ca2+ to induce phosphodiesterase activation. Activator protein requires 1.2 x 10(6) M and troponin-C, 1.9 X 10(6) M free Ca2+ for half-maximal stimulation of phosphodiesterase. The biological cross-reactivity of these proteins supports the sequence homology recently reported by Watterson et al. (Watterson, D.M., Harrelson, W.G., Keller, P.M., Sharief, F., and Vanaman, T.C. (1976) J.Biol. Chem. 251, 4501-4513). In addition, this preliminary study suggests that this nonmuscle troponin-C-like protein potentially may function in other Ca2+-regulated cellular events in addition to its moculation of cyclic nucleotide levels.     

Comparison of properties of Ca2+ release channels between rabbit and frog skeletal muscles

Biochemical investigation of Ca2+ release channel proteins has been carried out mainly with rabbit skeletal muscles, while frog skeletal muscles have been preferentially used for physiological investigation of Ca2+ release. In this review, we compared the properties of ryanodine receptors (RyR), Ca2+ release channel protein, in skeletal muscles between rabbit and frog. While the Ryr1 isoform is the main RyR of rabbit skeletal muscles, two isoforms, - and -RyR which are homologous to Ryr1 and Ryr3 isoforms in mammals, respectively, coexist as a homotetramer in a similar amount in frog skeletal muscles. The two isoforms in an isotonic medium show very similar property in [3H]ryanodine binding activity which is parallel to Ca2+-induced Ca2+ release (CICR) activity, and make independent contributions to the activities of the sarcoplasmic reticulum. CICR and [3H]ryanodine binding activities of rabbit and frog are qualitatively similar in stimulation by Ca2+, adenine nucleotide and caffeine, however, they showed the following quantitative differences. First, rabbit RyR showed higher Ca2+ affinity than the frog. Second, rabbit RyR showed higher activity in the presence of Ca2+ alone with less stimulation by adenine nucleotide than the frog. Third, rabbit RyR displayed less enhancement of [3H]ryanodine binding by caffeine in spite of having a similar magnitude of Ca2+ sensitization than the frog, which may explain the occasional difficulty by researchers to demonstrate caffeine contracture with mammalian skeletal muscles. Finally, but not least, rabbit RyR still showed marked inhibition of [3H]ryanodine binding in the presence of high Ca2+ concentrations in the 1 M NaCl medium, while frog RyR showed disinhibition. Other matters relevant to Ca2+ release were also discussed.     

Effect of adenosine 3′,5′-(cyclic)-monophosphate on the synthesis of progestational steroids by rabbit ovarian tissue in vitro

1. Adenosine 3',5'-(cyclic)-monophosphate (3',5'-AMP) stimulates the synthesis of progestational steroids by rabbit ovarian tissue in vitro. 2. Other adenosine phosphates fail to increase steroidogenesis. 3. The ratio of 20alpha-hydroxypregn-4-en-3-one to progesterone, the maximal response of the tissue, and the responses of separated corpora lutea and interstitial tissue produced by luteinizing hormone are closely paralleled by 3',5'-AMP. 4. In tissues maximally stimulated by luteinizing hormone, 3',5'-AMP fails to produce an additional response. 5. The addition of theophylline, an inhibitor of phosphodiesterase, potentiates the effects of 3',5'-AMP and also luteinizing hormone. 6. The results obtained suggest that 3',5'-AMP is a mediator of the action of luteinizing hormone on progestational steroid synthesis by rabbit ovarian tissue.