Isolation and purification of rabbit adrenal norephinephrine N-methyl transferase isozymes

Kinetic and pH studies on human phenylethanolamine N-methyltransferase

Phenylethanolamine N-methyltransferase (PNMT) catalyzes the conversion of norepinephrine (noradrenaline) to epinephrine (adrenaline) while, concomitantly, S-adenosyl-L-methionine (AdoMet) is converted to S-adenosyl-L-homocysteine. This reaction represents the terminal step in catecholamine biosynthesis and inhibitors of PNMT have been investigated, inter alia, as potential antihypertensive agents. At various times the kinetic mechanism of PNMT has been reported to operate by a random mechanism, an ordered mechanism in which norepinephrine binds first, and an ordered mechanism in which AdoMet binds first. Here we report the results of initial velocity studies on human PNMT in the absence and presence of product and dead end inhibitors. These, coupled with isothermal titration calorimetry and fluorescence binding experiments, clearly shown that hPNMT operates by an ordered sequential mechanism in which AdoMet binds first. Although the logV pH-profile was not well defined, plots of logV/K versus pH for AdoMet and phenylethanolamine, as well as the pKi versus pH for the inhibitor, SK&F 29661, were all bell-shaped indicating that a protonated and an unprotonated group are required for catalysis.

Properties of a partially purified phosphodimethylethanolamine methyltransferase from rat brain cytosol

The presence of cytosolic S-adenosylmethionine-dependent N-methyltransferase(s) activity(ies) capable of converting phosphoethanolamine into phosphocholine has been recently demonstrated in the rat brain. At least two enzymes are involved in the methylation of phosphoethanolamine to phosphocholine and these are separable by ammonium sulphate fractionation. The enzyme catalysing the last step of this methylation process is present in the 50-80% ammonium sulphate fraction. A 220-fold purified enzyme has been obtained with sequentially employed Q-Sepharose fast flow and octyl-Sepharose CL4B column chromatography. The maximum enzyme activity was at pH 9.5. The Km values for S-adenosylmethionine, the methyl donor, and phosphodimethylethanolamine, the methyl acceptor, were 125 microM and 750 microM respectively. This phosphodimethylethanolamine N-methyltransferase was found to be calcium-dependent, with a 4-fold increase in activity at 0.5 mM-CaCl2. S-Adenosylhomocysteine at 0.5 mM caused 100% inhibition of the activity. The effects of various structural analogues on the phosphodimethylethanolamine N-methyltransferase activity were also investigated and these results suggest that the enzyme is specific to the substrate. These results provide evidence for the existence of the pathway for the methylation of phosphoethanolamine to phosphocholine in rat brain cytosol.

Characterization of recombinant bovine phenylethanolamine N-methyltransferase expressed in a mouse C127 cell line

Bovine phenylethanolamine N-methyltransferase (PNMT) cDNA was inserted into a bovine papilloma virus-based expression vector and used to transfect a mouse C127 cell line. The resultant recombinant bovine PNMT was characterized biochemically and immunochemically. Recombinant bovine PNMT activity, like the native bovine enzyme, was enhanced by phosphate ion in a concentration-dependent manner. Their molecular weights were shown to be identical by Western blot analysis. Antibodies raised against native bovine adrenal PNMT equally immunoprecipitated the activity of the recombinant and native enzymes. In addition, double immunodiffusion analysis showed a single precipitin line of confluence with both enzyme preparations, indicating immunological identity of native and recombinant bovine PNMT. These antibodies immunostained the recombinant enzyme protein in transfected cells and in their neurite-like processes. In addition, in situ hybridization with the bovine PNMT cDNA probe resulted in a labelling pattern similar to the immunostaining. The recombinant bovine PNMT as the native bovine enzyme exist in multiple-charge forms, but only one form is predominant. Taken together, our results suggest that recombinant bovine PNMT, expressed from bovine PNMT cDNA in a mouse cell line is enzymatically active and shares many common features with native bovine adrenal PNMT.

Possible mechanism of action of SKF 64139 in vivo on rat adrenal and brain phenylethanolamine N-methyltransferase activity

SKF 64139, a specific inhibitor of the epinephrine-synthesizing enzyme, phenylethanolamine N-methyltransferase (PNMT), has been widely used as a pharmacological tool for studying the characteristics of epinephrine-containing neurons. However, the mechanism of action of this drug on PNMT in vivo has not been fully elucidated. In the present study, we traced changes of PNMT activity in rat adrenal glands and medulla oblongata between 1 and 48 hr after intraperitoneal injection of SKF 64139 (50 mg/kg body wt). Within 1 hr, enzyme activity in both tissues decreased to 10% of the respective control value. However, starting at 4 hr, activity gradually recovered from the inhibition and completely returned to the respective control level by 48 hr. Removal of the inhibitor by dialysis substantially restored the adrenal enzyme activity in 1, 2 and 4 hr groups and completely returned it to control levels in 18 and 48 hr groups. A similar pattern also seemed to hold with brain extracts. The profiles of immunotitration curves, using dialyzed tissue extracts and specific antibodies to bovine adrenal PNMT, clearly indicate that, even after dialysis, a substantial amount of inactive enzyme was present in tissue extracts from 1, 2 and 4 hr groups. In contrast, by 18 hr a very small amount of inactive enzyme was present. Throughout the experimental periods there was no noticeable differences among the control and the experimental groups in the number or intensity of immunocytochemical stained neurons with PNMT antibodies of the C1 area of ventrolateral medulla. Judging from the data obtained by dialysis, immunochemical titration and immunocytochemical staining, recovery of PNMT activity following its inhibition by SKF 64139 was not due to irreversible inhibition of the enzyme followed by new enzyme synthesis. Instead, reversible binding of inhibitor to PNMT and its release were responsible for recovery. PNMT from the 1, 2 and 4 hr groups resisted further in vitro inhibition by SKF 64139 because the residual inhibitor was probably still bound to the enzyme.

Characterization of the isozymes of bovine adrenal medullary phenylethanolamine N-methyltransferase

Bovine adrenal medullary phenylethanolamine N-methyltransferase (EC 2.1.1.28) has been purified to apparent homogeneity. The enzymatically active monomer has a relative molecular weight of 30,000 and can be separated into at least four active charged isozymes. These isozymes, designated PNMT-1, PNMT-2, PNMT-3 and PNMT-4, have isoelectric points of 5.1, 5.2, 5.3 and 5.4, respectively. Kinetic parameters have been determined for each isozyme. The Kms for phenylethanolamine range from 11.9 to 45.9 microM; the Kms for S-adenosylmethionine range from 1.13 to 1.47 microM; and the Kis for the competitive inhibitor, S-adenosylhomocysteine, range from 0.12 to 0.22 microM. For isozymes PNMT-1 and PNMT-4, and Kms for S-adenosylhomocysteine are not significantly different. Vmax values for all of the isozymes do not change significantly in the presence of S-adenosylhomocysteine. Treatment of the purified isozymes with various endo- and exoglycosidases does not alter electrophoretic mobility. Hence, carbohydrate substitution must be minimal. No high mannan, complex sugars or terminal N-acetylglucosamine residues are present. The absence of carbohydrate is further supported by the inability of Schiff-periodic acid to stain the protein. Limited thermolysin digests of each isozyme show distinct peptide cleavage products. In conjunction with the kinetic and glycosylation data, this suggests that the isozymes of phenylethanolamine N-methyltransferase may be primary structural variants.

Species-specific charge forms of phenylethanolamine N-methyltransferase

Isoelectric points (pI) of phenylethanolamine N-methyltransferase (PNMT) from two species, cow and rat, were determined by chromatofocusing. Bovine PNMT has 5 different charge isozymes ranging from 5.4 to 6.2. In contrast, rat PNMT has only a single charge form of pI 4.8. There is no common isoelectric point of PNMT from both species and there is no tissue variation in the isoelectric point.

Activation of adrenal phenylethanolamine N-methyltransferase by phosphate

Bovine adrenal phenylethanolamine N-methyltransferase is activated by inorganic phosphate. Addition of potassium phosphate to the enzyme assay mixture increased the enzyme activity 5- to 8-fold depending on the purity of the enzyme. Neuraminidase treatment of the enzyme decreased the activation by one half. The activity of the enzyme was also increased by other negatively charged ions, including chloride and sulfate. However, the activation was less than 3-fold, and pretreatment with neuraminidase did not alter the degree of activation. The high degree of activation was specific to bovine adrenal enzyme. Rat adrenal enzyme was activated less than 2-fold by phosphate and not affected at all by chloride and sulfate. These results suggest that bovine adrenal phenylethanolamine N-methyltransferase is a glycosylated protein, containing sialic acid moieties, and that this carbohydrate moiety plays a role in the activation of this enzyme. These results are also consistent with our previous finding [D. H. Park, E. E. Baetge, B. B. Kaplan, V. R. Albert, D. J. Reis and T. H. Joh, J. Neurochem. 38, 410 (1982)] that the difference in characteristics between bovine and rat adrenal phenylethanolamine N-methyltransferase is due to posttranslational modification of the enzyme.

Phenylethanolamine N-methyltransferase: notes on its purification from bovine adrenal medulla and separation from protein carboxymethyltransferase

Standard procedures for the purification of phenylethanolamine N-methyltransferase were modified by the addition of an affinity chromatography step utilizing immobilized S-adenosyl-L-homocysteine and by use of preparative isoelectric focusing. Enzyme derived from bovine adrenal medullae was bound to S-adenosyl-L-homocysteine agarose, and could be eluted with 0.1 M NaCl. Concentrations of S-adenosyl-L-methionine as high as 10 mM were ineffective in eluting the enzyme. Preparative isoelectric focusing of bovine phenylethanolamine N-methyltransferase showed a single peak with the pI = 4.95. The potential use of immobilized S-adenosyl-L-homocysteine in the differential separation of phenylethanolamine N-methyltransferase from other methyltransferase enzymes is discussed.

Multiple forms of glutamate decarboxylase in porcine brain

Three forms of glutamate decarboxylase from hog brain (termed alpha-, beta-, and gamma-GAD) were separated by hydrophobic interaction chromatography on phenyl-Sepharose, by isoelectric focusing, and by polyacrylamide gel electrophoresis. When rechromatographed on phenyl-Sepharose, each form migrated as a single entity, indicating that the forms are not readily interconvertible. The three forms are not different-sized aggregates of one form, since all three have the same approximate molecular weight (100,000) as determined by Sephadex G-200 chromatography. The pIs of the three forms separated by phenyl-Sepharose were determined by isoelectric focusing. The values obtained (5.3, 5.5, and 5.8 for alpha-, beta-, and gamma-GAD, respectively) were comparable to the pIs of the three peaks of activity observed upon focusing of enzyme that had been subjected to phenyl-Sepharose chromatography. These results indicate that phenyl-Sepharose chromatography and isoelectric focusing separate the same three components. When synaptosomal extracts were analyzed by phenyl-Sepharose chromatography without intervening purification steps, all three forms were present, but the proportion of beta-GAD was somewhat higher and that of gamma-GAD somewhat lower than in the usual preparations.

Tetrahydroisoquinolinecarboxylic acids and regulation of phenylethanolamine N-methyltransferase in cultured adrenal medulla

Explants of rat adrenal medullae, cultured for up to 48 hr in the presence of 3',4'-deoxynorlaudanosolinecarboxylic acid (DNLCA) or alpha-methyltyrosine, exhibited a 69% increase in phenylethanolamine N-methyltransferase (PNMT) activity as measured in dialyzed homogenates. A related tetrahydroisoquinoline, norlaudanosolinecarboxylic acid (NLCA), when added to the medium did not elevate PNMT activity. No increase in the amount of PNMT was detected by immunochemical titration of homogenates from DNLCA-treated cultured medulla, nor were there changes in rates of synthesis or degradation of the enzyme. Although DNLCA is an inhibitor of tyrosine 3-monooxygenase, it had no effect on PNMT activity when added directly to an incubation mixture in vitro. Kinetic analyses of dialyzed homogenates from explants cultured in the presence of DNLCA revealed that the Vmax of PNMT was higher than that of control tissue. There was no decrease in Km after DNLCA treatment. The increase in PNMT activity appears to be a compensatory response to depletion of medullary catecholamines by DNLCA or alpha-methyltyrosine.

Different forms of adrenal phenylethanolamine N-methyltransferase: species-specific posttranslational modification

Phenylethanolamine N-methyltransferase was purified from rat and cow adrenal glands. The enzymes from the two species have the same molecular weight of 31,000, but differ in electrophoretic mobility. During polyacrylamide gel electrophoresis, the rat form migrates faster than the bovine form. Antibodies to bovine enzyme precipitated equally well the rat and cow form of the enzyme, but antibodies against rat enzyme precipitated poorly the bovine form. In contrast, both antibodies recognized a similar protein in the in vitro translation products of poly(A+)mRNA isolated from cow adrenal glands. The results suggest that the primary protein structure of rat and bovine enzyme is similar and that differences in electrophoretic mobility are due to posttranslational modification of the enzyme molecule.