Significance of the variation in isozymes of liver lactate dehydrogenase with thermal acclimation in goldfish--II. Effect of pH.

1. Effect of pH on liver lactate dehydrogenase (LDH) and its isozymes was examined in the goldfish acclimated to different temperatures and some purification of the LDH was attempted. 2. The optimal pH and the Km value at 30 degrees C of the enzyme were independent of acclimation temperature. 3. the optimal pH of isozyme was more basic in the order of LDH-1, LDH-2, LDH-3, LDH-4 and LDH-5. Km values of isozymes at 30 degrees C were higher in the order of LDH-1, LDH-3 and LDH-5. 4. There was no change in the enzyme activity during thermal acclimation.     

Energy enzymes of the flight muscle of the house sparrow, Passer domesticus--III. Comparative physical properties of heart and flight muscle lactate dehydrogenase

Purification of heart (LDH-4) and flight muscle (LDH-2 and LDH-3) lactate dehydrogenase isoenzymes from the house sparrow, Passer domesticus, has been accomplished. Although these isoenzymes electrophoretically migrate reversed to most other vertebrate LDH isoenzymes, comparison of the amino acid compositions of LDH-4 and LDH-2-LDH-3 fails to reveal the basis for their reversed electrophoretic migration. Amino acid compositions did reveal mol. wts between 141,000-142,000 as well as vp of 0.744 ml/g (LDH-4) and 0.745 ml/g (LDH-2-LDH-3). SDS-gel electrophoresis yielded single bands for each preparation with mol. wts of 35,000 suggesting that LDH in this species exists as a tetramer. LDH-4 has a lower Km for both pyruvate (0.005 mM) and NADH (0.002 mM) than does LDH-2-LDH-3 (0.062 mM for pyruvate, 0.013 mM for NADH).     

Expression of lactate dehydrogenase isozyme 5 (LDH-5) in cultured mouse blastocysts in the absence of implantation and outgrowth

Extensive extraction studies with Triton X-100 revealed only LDH-1 (B₄) but no trace of LDH-5 (A₄) in one-cell and two-cell mouse and rat embryos. The LDH isozyme pattern of preimplantation mouse embryos changes from the maternally inherited B subunit isozyme (LDH-1) to a pattern dominated by LDH-5 when mouse blastocysts are cultured under conditions that prevent hatching but allow trophoblast giant cell transformation. During differentiation of mouse blastocysts in vitro, implantation is therefore not essential for the appearance of the A subunit form of LDH (LDH-5) coded for by the embryonic genome. Mechanisms controlling the expression of LDH-5 in mouse blastocysts during in vivo development are discussed.This work was supported by grants of the Deutsche Forschungsgemeinschaft awarded to the Sonderforschungsbereich 29—Embryonale Entwicklung and Differenzierung.     

高分子量和低分子量尿激酶的分离纯化及动力学性质研究

人尿激酶粗品经苯甲脒亲和柱纯化和Protein-PahSP柱分离后,得到两种分子量的尿激酶（UK）,即高分子量尿激酶（HUK）和低分子量尿激酶（LUK）．采用考马斯亮蓝法测定蛋白质浓度,纤维蛋白平板法测定活力,测得HUK比活为2.9×105IU／mg蛋白,LUK为3.5×105IU／mg蛋白,活力回收为70％以上．经SDS-PAGE鉴定,HUK和LUK均是单一条带,分子量分别为54kD和33kD．HUK和LUK水解显色底物S2444的动力学常数,分别测得HUK的Km为64μmol/L,Kcat为15s-1,LUK的Km为49μmol／L,kcat为13s-1,LUK的催化效率（Kcat／Km）稍高于HUK．     

Lactate dehydrogenase isoenzymatic analysis of murine lymphocyte populations: a parameter of cellular differentiation.

The lactate dehydrogenase isoenzyme pattern has been determined in different murine lymphocytic cell populations. In each cell population, the LDH activity was predominantly found in the LDH-4 and LDH-5 fractions. The percentage LDH-5 activity was significantly higher in B cells than in T cells. The same is true for lymphocytes from the spleen versus lymph node lymphocytes. The percentage LDH-5 activity is significantly higher in peripheral T lymphocytes than in thymocytes. Enrichment of the more mature thymocytes of the thymocyte cell pool by either cortisone treatment in vivo or gradient centrifugation on bovine serum albumin (BSA) results in a decrease of LDH-1 and LDH-2 fractions. In the cortisone-treated group, the shift in the LDH pattern is accompanied by a significant increase of LDH-5 and LDH-4 fractions, whereas in the BSA group only the LDH-4 fraction increases.     

The effects of enzyme concentration on the kinetic behavior of adrenal 21-hydroxylase and 11beta-hydroxylase in the pregnant rat.

Experiments were designed to study the kinetic behavior of 21-hydroxylase and 11beta-hydroxylase as a function of enzyme concentration (Et) during proestrus, dasy 5 (D5), 12 (D12), and 22 (D22) of pregnancy, and within 24 h post-partum. The enzymes were prepared from rat adrenal microsomes and mitochondria, respectively. The experiments consisted of measuring the initial velocity of each reaction for a series of substrate concentrations at three fixed Et. Double reciprocal plots were constructed and the slope (Km/Vmax) of each line estimated. Variation in the value of the slope as a function of enzyme dilution would predict the presence of an endogenous effector. The kinetic behavior of 21-hydroxylase was not altered throughout the range of Et (10-100 microgram protein) at any of the reproductive stages. In contrast, kinetic behavior of 11beta-hydroxylase was clearly dependent upon Et. Dilution of the enzyme preparation (25-200 microgram of protein) increased the slope of the double reciprocal plot at all reproductive stages, thus suggesting that an activator substance may be present within the mitochondrial preparation. A secondary plot of the slope (Km/Vmax) versus Et described a power function (Km/Vmax = a [Et]b) with the greatest rate of change in Km/Vmax occurring at low values of Et. The rate of change in Km/Vmax per mg rise in mitochondrial protein at all dilutions of enzyme was greatest for proestrus and post-partum, followed by D22 greater than D12 greater than D5. In addition, repeated washing of the enzyme preparation at 4 degrees C increased Km/Vmax to a greater extent at all Et than did the control preparation. These findings suggest the presence of a diffusible endogenous activator of 11beta-hydroxylase whose influence decreases markedly at D5 and D12. On the other hand, there is no evidence to suggest the presence of a diffusible endogenous effector for 21-hydroxylase.     

High Km soluble 5'-nucleotidase from human placenta. Properties and allosteric regulation by IMP and ATP

A human placental soluble "high Km" 5'-nucleotidase has been separated from "low Km" 5'-nucleotidase and nonspecific phosphatase by AMP-Sepharose affinity chromatography. The enzyme was purified 8000-fold to a specific activity of 25.6 mumol/min/mg. The subunit molecular mass is 53 kDa, and the native molecular mass is 210 kDa, suggesting a tetrameric structure. Soluble high Km 5'-nucleotidase is most active with IMP and GMP and their deoxy derivatives. IMP is hydrolyzed 15 times faster than AMP. The enzyme has a virtually absolute requirement for magnesium ions and is regulated by them. Purine nucleoside 5'-triphosphates strongly activate the enzyme with the potency order dATP greater than ATP greater than GTP. 2,3-Diphosphoglycerate activates the enzyme as potently as ATP. Three millimolar ATP decreased the Km for IMP from 0.33 to 0.09 mM and increased the Vmax 12-fold. ATP activation was modified by the IMP concentration. At 20 microM IMP the ATP-dependent activation curve was sigmoidal, while at 2 mM IMP it was hyperbolic. The A0.5 values for ATP were 2.26 and 0.70 mM, and the relative maximal velocities were 32.9 and 126.0 nmol/min, respectively. Inorganic phosphate shifts the hyperbolic substrate velocity relationship for IMP to a sigmoidal one. With physiological concentrations of cofactors (3 mM ATP, 1-4 mM Pi, 150 mM KCl) at pH 7.4, the enzyme is 25-35 times more active toward 100 microM IMP than 100 microM AMP. These data show that: (a) soluble human placental high Km 5'-nucleotidase coexists in human placenta with the low Km enzyme; (b) under physiological conditions the enzyme favors the hydrolysis of IMP and is critically regulated by IMP, ATP, and Pi levels; and (c) kinetic properties of ATP and IMP are each modified by the other compound suggesting complex interaction of the associated binding sites.     

Significance of the variation in isozymes of liver lactate dehydrogenase with thermal acclimation in goldfish--I. Thermostability and temperature dependency.

1. Total and isozyme properties as well as isozyme pattern were examined in liver lactate dehydrogenase (LDH) from goldfish acclimated to different temperatures. 2. LDH of warm-acclimated fish were thermostable and exhibited higher Q10 in low temperature range as compared with that of co ld-acclimated fish. 3. The relative activities of LDH-1, LDH-2 and LDH-3, which were more thermostable, increased and LDH-4 and LDH-5, which were more heat sensitive, decreased during warm acclimation. Q10 in the low temperature range for LDH-5 was lower than that for LDH-1.     

Lactate dehydrogenase isoenzymatic spectrum in preimplantation murine embryos

The spectrum of LDH isozymes was studied using different concentration of X-100 triton for enzyme extraction from the eggs and embryos, different systems of electrophoresis and different methods of staining electrophoregrams. Similar results were obtained in all cases. The mouse eggs and embryos were shown to contain only LDH-1; the treatment of oviducal fluid with X-100 triton strengthened the activity of LDH-4 and LDH-5. The detection of the activity of LDH-5 and hybrid isozymes appears to be related to the presence of oviducal fluid in the sample under study.     

Giardia lamblia: characterization of proteinase activity in trophozoites

The proteinase activity of Giardia lamblia trophozoites, Portland 1 strain, was characterized with respect to substrate specificities and inhibitor sensitivities. Proteinase activity with urea-denatured hemoglobin (UDH), alpha-N-benzoyl-DL-arginine-2-naphthylamide (BANA), and alpha-N-benzoyl-argininamide (BAA) as substrates exhibited pH optima of 5.8, 3.8, and 5.0, respectively. For BANA, the apparent Km was 0.20 mM and the Vmax was 2.56 microM. For BAA, the apparent Km was 4.0 mM and the Vmax was 8.69 microM. Dithiothreitol (DTT, 5 mM) enhanced proteinase activity threefold for UDH, fourfold for BAA, and fivefold for BANA. Iodoacetamide, L-tosylamide-2-phenylethyl chloromethyl ketone (TPCK), and N-alpha-p-tosyl-L-lysine chloromethyl ketone (TLCK), each at 1 mM, inhibited proteinase activity by greater than 90% with BANA and BAA. Iodoacetamide inhibited proteinase activity by 35% with UDH; TPCK and TLCK inhibited activity greater than 70% with UDH. Activity on BAA was inhibited by 91% with Zn2+ and activity on UDH was inhibited by 30% with Cu2+. Virtually complete inhibition of proteinase activity on BANA and BAA was obtained with leupeptin and chymostatin at 1 microgram/ml. Pepstatin A, chelators, and other heavy metals had no apparent effect on proteinase activity. Two polypeptide bands (ca. 105 and 40 kDa) indicative of proteinase activity were visualized by sodium dodecyl sulfate-gelatin polyacrylamide gel electrophoresis. The 105 kDa band was visible over the pH range of 4 to 7, but with greater intensity from pH 5 to 7. The 40 kDa band, while present at pH 5, was most intense at pH 6 and 7.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of high carbohydrate diet on serum lactate dehydrogenase isozyme pattern in Japanese young men

Changes in the activity of serum lactate dehydrogenase (LDH) and the serum LDH isozyme pattern in healthy young men given a high carbohydrate diet (480-636 g/day, 80% of the total energy) for 21 days were examined. Serum total LDH activity showed no significant change in four healthy young volunteers who received high carbohydrate diet for 21 days. However, the percentage of LDH-4 increased significantly (P less than 0.01) from 8.5 +/- 2.4 to 10.9 +/- 1.9% after 21 days, as did also the percentage of LDH-5 (P less than 0.01) from 5.1 +/- 1.9 to 10.7 +/- 2.9%. The percentage of M-type LDH activity increased 30% during the experiment (P less than 0.05). It is concluded from the results that the high carbohydrate intake affects the percentage of LDH-4 and LDH-5, but not the total serum LDH activity.     

Human lactic dehydrogenase as a marker for monitoring the growth of prostatic carcinoma in nude mice

Human lactate dehydrogenase (1,1,1,27-LDH) isoenzymes were determined in human prostatic carcinoma cell lines. The isoenzymes LDH-4/LDH-5 were identified and quantified in the serum of athymic mice bearing the human prostatic carcinoma cell lines PC-3, PC-3/M and DU 145 subcutaneously. The amounts of the slowly migrating isoenzymes released by the solid tumors correlated with increasing tumor volume and tumor weight. These correlations indicate that the human LDH-4/LDH-5 isoenzymes are useful parameters for monitoring the presence and growth of prostatic cancer in the athymic animal model.     

Lactate dehydrogenase isoenzymes in diabetic patients

Lactate dehydrogenase (LDH) isoenzyme profiles in human platelets and the sera of patients with type I and II diabetes mellitus and vascular complications, as well as normal subjects were measured utilizing a recently established, modified micromethod. LDH-3 was the predominating species in platelets (37.5 +/- 3.0%), with LDH-2, 1, 4 and 5 following in decreasing order of concentration. The LDH-3/LDH-4 ratio in platelets varied from 6.2 to 1.38. Type I and type II diabetic patients with vascular complications showed a significantly higher ratio for LDH-3/LDH-4 (3.99 +/- 1.20 for DM I, 2.16 +/- 0.25 for DM II patients) than the mean ratio for normal subjects (1.14 +/- 0.08). This platelet-specific LDH isoenzyme pattern may be the result of frequent in vivo platelet-vessel wall interactions in the diabetic patients whose platelets are known to be hyperaggregable in in vitro test systems. Since non-diabetic patients patients with vascular complications also displayed a similarly elevated LDH-3/LDH-4 ratio, a wider classification is preferable, although the measurement of the LDH isoenzyme pattern will be helpful in assessing diabetic vascular complications.     

Decreased activities of cyclic cytidine 3',5'-monophosphate phosphodiesterase in Morris hepatomas having varying growth rates

1. The activities of cyclic cytidine 3',5'-monophosphate (cCMP) phosphodiesterase in normal rat liver and host liver (bearing hepatoma 5123 t.c.(h)) were compared with those of three Morris hepatomas of varying growth rates. 2. The results show that the order of enzyme activity was as follows: normal liver = host liver greater than 7794A (slow growth rate) greater than 5123 t.c.(h) (intermediate growth rate) greater than 7800 (fast growth rate). 3. The enzyme had a pH optimal value of about 7.0 and an apparent Km for cCMP about 2.8 mM; its activity was slightly affected by the presence of calmodulin (100 micrograms/ml) and/or CaCl2 (100 microM), but showed variable responses to other cations (La3+, Mg2+, Mn2+, Zn2+, Fe2+, Na+ and K+).     

Steroid sulfatase in the human ovary and placenta: enzyme kinetics and phosphate inhibition.

In 5 placental homogenates the Km of steroid sulfatase for DHEA sulfate increased from 15.4 in Tris buffer to 26.8 microM in phosphate (both buffers 0.1 M, pH 7.4), P less than 0.05. In 3 pooled ovarian preparations the Km increased from 14.3 microM in Tris to 33.0 microM in phosphate, P less than 0.01. There was no significant difference between the ovarian and placental values for Km in either Tris or phosphate (P greater than 0.5), and the increase in the Km produced by phosphate in ovarian tissue was not significantly different from that in the placenta (P greater than 0.5). In the placentas the Vmax in Tris was 1420 pmol/min/mg protein and this fell to 523 pmol/min/mg protein in phosphate (P less than 0.005). The Vmax was 50-fold higher in the placenta than in the ovary in either Tris or phosphate (both P less than 0.001). In the ovary, the Vmax was 27.6 pmol/min/mg protein in Tris and 11.0 pmol/min/mg protein in phosphate (P less than 0.05). The reduction of Vmax produced by phosphate in the ovary was not significantly different from that in the placenta (P greater than 0.5). The slope of the 1/v vs 1/S plot (Km/Vmax) increased 4.7-fold in the placentas and 5.8-fold in the ovaries in phosphate over that in Tris (both P less than 0.001); the increase in the placentas was not significantly different from that in the ovaries (P greater than 0.5). Phosphate ion acts as a mixed inhibitor of both placental and ovarian steroid sulfatase.     

Protective effects of N-acetyl cysteine on lipid peroxide metabolism on isoproterenol-induced myocardial infarcted rats

The present study was designed to evaluate the preventive effects of N-acetyl cysteine on lipid peroxide metabolism in isoproterenol (ISO) induced myocardial infarcted rats. Male albino Wistar rats were pretreated with N-acetyl cysteine (5 and 10 mg/kg) daily for a period of 14 days. After the pretreatment period, ISO (100 mg/kg) was subcutaneously injected to rats twice at an interval of 24 h. Increased activities of serum creatine kinase, creatine kinase-MB, lactate dehydrogenase, and increased intensities of serum lactate dehydrogenase-isoenzyme bands (LDH-1, LDH-2) were observed in ISO-induced rats. The heart lipid peroxidation products were significantly increased, and the antioxidant system was significantly reduced in ISO-induced rats. Pretreatment with N-acetyl cysteine (5 and 10 mg/kg) to ISO-induced rats showed significant effects on all the biochemical parameters studied. Histopathological findings of the myocardium also showed the protective role of N-acetyl cysteine in ISO-induced rats. Furthermore, in vitro study confirmed the potent-free radical scavenging activity of N-acetyl cysteine. The effect at a dose of 10 mg/kg of N-acetyl cysteine was more pronounced than the dose, 5 mg/kg. The results of our study show that N-acetyl cysteine protects the heart against ISO-induced myocardial infarction by its free radical scavenging effect.     

Recognition of ferric catecholates by FepA

Escherichia coli FepA transports certain catecholate ferric siderophores, but not others, nor any noncatecholate compounds. Direct binding and competition experiments demonstrated that this selectivity originates during the adsorption stage. The synthetic tricatecholate Fe-TRENCAM bound to FepA with 50- to 100-fold-lower affinity than Fe-enterobactin (FeEnt), despite an identical metal center, and Fe-corynebactin only bound at much higher concentrations. Neither Fe-agrobactin nor ferrichrome bound at all, even at concentrations 10(6)-fold above the Kd. Thus, FepA only adsorbs catecholate iron complexes, and it selects FeEnt among even its close homologs. We used alanine scanning mutagenesis to study the contributions of surface aromatic residues to FeEnt recognition. Although not apparent from crystallography, aromatic residues in L3, L5, L7, L8, and L10 affected FepA's interaction with FeEnt. Among 10 substitutions that eliminated aromatic residues, Kd increased as much as 20-fold (Y481A and Y638A) and Km increased as much as 400-fold (Y478), showing the importance of aromaticity around the pore entrance. Although many mutations equally reduced binding and transport, others caused greater deficiencies in the latter. Y638A and Y478A increased Km 10- and 200-fold more, respectively, than Kd. N-domain loop deletions created the same phenotype: Delta60-67 (in NL1) and Delta98-105 (in NL2) increased Kd 10- to 20-fold but raised Km 500- to 700-fold. W101A (in NL2) had little effect on Kd but increased Km 1,000-fold. These data suggested that the primary role of the N terminus is in ligand uptake. Fluorescence and radioisotopic experiments showed biphasic release of FeEnt from FepA. In spectroscopic determinations, k(off1) was 0.03/s and k(off2) was 0.003/s. However, FepAY272AF329A did not manifest the rapid dissociation phase, corroborating the role of aromatic residues in the initial binding of FeEnt. Thus, the beta-barrel loops contain the principal ligand recognition determinants, and the N-domain loops perform a role in ligand transport.     

Purification and some properties of isozymes 1 and 5 of lactate dehydrogenase from fox heart and skeletal muscles]

An improved method is described for the isolation of isozymes 1 and 5 of lactate dehydrogenase (LDH) from heart and skeletal muscles of foxes. The method includes salt fractionation with ammonium sulphate, chromatography on DEAE- and CM-celluloses and affinity chromatography on AMP-Sepharose. The preparations of LDH isozymes 1 and 5 turned out to be homogeneous both in 7,5% polyacrylamide gel electrophoresis and under immunodiffusion analysis. It is shown that the pH optimum for LDH-1 is 10.2-10.4 for LDH-5 it is 9.5-9.6 in the case of the direct reaction, and the pH optimum is 7.6-7.8 for LDH-1 and 7.3-7.4 for LDH-5 in the case of reverse reaction. The values of Mikhaelis constants were determined for substrates and coenzymes in direct and reverse reactions. It is found that the excess of lactate and pyruvate causes substrate inhibition of both LDH-1 and LDH-5. The activities of LDH-1 and LDH-5 showed an unexpected similar sensitivity to the inhibitory effect of high pyruvate concentrations.     

Selective inactivation of lactate dehydrogenase of rat tissues by sodium deoxycholate.

Soluble lactate dehydrogenase (EC 1.1.1.27) extracted from brain, skeletal and cardiac muscle and liver of rats, and purified isoenzymes LDH-1 and LDH-5, were incubated with sodium deoxycholate. Deoxycholate almost totally inactivated isoenzyme LDH-5 (A4), whereas it left isoenzyme LDH-1 (B4) unaffected. Tissue lactate dehydrogenase was inactivated to different degrees depending on the origin of the enzyme. Electrophoretic isoenzyme studies of tissue lactate dehydrogenase showed the loss of activity to be quantitatively related to the overall percentage of subunit A distributed among the homotetramer LDH-5 and the heterotetramers LDH-2, LDH-3 and LDH-4. It was concluded that subunit A of lactate dehydrogenase interacts selectively with deoxycholate, irrespective of its association with subunit B. Distinct changes in electrophoretic mobilities of deoxycholate-treated isoenzymes strongly indicated an indiscriminate binding of deoxycholate by all LDH isoenzymes, probably through hydrophobic interactions. The results suggest that the inactivation of the enzyme is non-competitive, but the basis of the selectivity of deoxycholate towards subunit A is not known at present.     

Cyclic 3',5'-AMP phosphodiesterase of rabbit aorta.

Cyclic AMP and cyclic GMP phosphodiesterase activities (3' : 5'-cyclic AMP 5'-nucleotidohydrolase, EC 3.1.4.17) were demonstrated in the isolated intima, media, and adventitia of rabbit aorta. The activity for cyclic AMP hydrolysis in the intima was 2.7-fold higher than that for cyclic GMP hydrolysis. The activity for cyclic AMP hydrolysis in the media was approximately equal to that for cyclic GMP hydrolysis, but in the adventitia, cyclic GMP hydrolytic activity was 2.1-fold higher than cyclic AMP hydrolytic activity. Distribution of the activator of the phosphodiesterase was studied in the three layers. Each layer contained the activator. The activator was predominantly localized in the smooth muscle layer (the media). The effect of the activator and Ca2+ on the media cyclic AMP and cyclic GMP phosphodiesterase was also briefly studied. The activity of the cyclic GMP phosphodiesterase was stimulated by micromolar concentration of Ca2+ in the presence of the activator. However, the activity of the cyclic AMP phosphodiesterase was not significantly stimulated by Ca2+ up to 100 muM in the presence of the activator. Above 90% of cyclic nucleotide phosphodiesterase activity in the whole aorta was found to be derived from the media. A major portion (60-70%) of the media enzyme was found in 105 000 times g supernatant. Cyclic AMP phosphodiesterase in the supernatant was partially purified through Sepharose 6B column chromatography and partially separated from cyclic GMP phosphodiesterase. Using a partially purified preparation from the 105 000 times g supernatant the main kinetic parameters were specified as follows: 1) The pH optimum was found to be about 9.0 using Tris-maleate buffer. The maximum stimulation of the enzyme by Mg2+ was achieved at 4mM of MgC12. 2) High concentration of cyclic GMP (0.1 mM) inhibited noncompetitively the enzyme activity, and the activity was not stimulated at any tested concentration of cyclic GMP. 3) Activity-substrate concentration relationship revealed a high affinity (Km equals 1.0 muM) and low affinity (Km equals 45 muM) for cyclic AMP. The homogenate and 105 000 times g supernatant of the media also showed non-linear kinetics similar to the Sepharose 6B preparation and their apparent Km values for cyclic AMP hydrolysis were 1.2 muM and 36-40 muM and an enzyme extracted by sonication from 105 000 times g precipitate also exhibited non-linear kinetics (Km equals 5.1 muM and 70 muM). 4) Papaverine exhibited much stronger inhibition on the aorta cyclic AMP phosphodiesterase (50% inhibition of the intima enzyme, I5 o at 0.62 muM, I5 o of the media at 0.62 muM and I5 o of the adventitia at 1.0 muM) than on the brain (I5 o at 8.5 muM) and serum (I5 o at 20 muM) cyclic AMP phosphodiesterase, while theophylline inhibited these enzymes similarly. However, cyclic GMP phosphodiesterases in all tissues examined were inhibited similarly, not only by theophylline but also by papaverine.