Peptidylglycine alpha-amidating monooxygenase (PAM, EC 1.14.17.3) catalyzes the formation of alpha-amidated peptides from their glycine-extended precursors, thus playing a key role in the processing of peptide neurohormones. We now report that PAM readily catalyzes three alternate monooxygenase reactions--sulfoxidation, amine N-dealkylation, and O-dealkylation. Thus, (4-nitrobenzyl)thioacetic acid is converted to the analogous sulfoxide, N-(4-nitrobenzyl)glycine is converted to 4-nitrobenzylamine and glyoxylate, and [(4-nitrobenzyl)oxy]acetic acid is converted to 4-nitrobenzyl alcohol and glyoxylate. All these new activities display the characteristics expected for the normal PAM-catalyzed reductive oxygenation pathway and produce an equimolar amount of glyoxylate together with the heteroatom-containing dealkylation products. The ester [(4-methoxybenzoyl)oxy]acetic acid is not a PAM substrate, but is instead a good competitive inhibitor (KI = 0.48 mM). In addition, we report that the olefinic substrate analogues trans-benzoylacrylic acid and 4-phenyl-3-butenoic acid are potent time-dependent inactivators of PAM, with inactivation exhibiting the characteristics expected for mechanism-based inhibition. Monoethyl fumarate is also a time-dependent inactivator of PAM. Finally, we introduce several small non-peptide substrates for PAM by demonstrating that PAM catalyzes the transformation of hippuric acid and several ring-substituted derivatives to the corresponding benzamides and glyoxylic acid, with the most facile substrate of this class being 4-nitrohippuric acid. These compounds are the smallest amide substrates yet reported for PAM, and it is thus apparent that only the minimal structure of an acylglycine is required for PAM-catalyzed oxygenative amidation.

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We report here the isolation of a novel enzyme from bovine neurointermediate pituitary which catalyzes the conversion of alpha-hydroxybenzoylglycine to benzamide. This enzyme, termed HGAD (alpha-hydroxyglycine amidating dealkylase), is a soluble protein with an apparent molecular mass of 45 kDa and no apparent cofactor requirement. Addition of HGAD to purified neurointermediate pituitary PAM (peptidylglycine alpha-amidating monooxygenase, EC 1.14.17.3) increases the rate of formation of amide products by an order of magnitude. Sequential additions of PAM and HGAD gave results consistent with PAM first catalyzing the formation of an intermediate that is subsequently, in a separate reaction, converted by HGAD to the final amide product. Experiments with olefinic inactivators demonstrate that HGAD is not required for turnover-dependent inactivation of PAM and, correspondingly, that HGAD activity is not affected by inactivators of PAM. As expected, HGAD has no effect on the rate of PAM-catalyzed sulfoxidation, where a reaction analogous to that occurring during amidation of glycine-extended substrates is not possible. On the basis of these results, we propose that peptide C-terminal amidation in neurointermediate pituitary is a two-step process, with PAM first catalyzing the conversion of a glycine-extended peptide to the alpha-hydroxyglycine derivative, which is in turn converted to the final amide product by HGAD.

In an initial communication [May, S. W., Mueller, P. W., Padgette, S. R., Herman, H. H., & Phillips, R. S. (1983) Biochem. Biophys. Res. Commun. 110, 161-168], we reported that 1-phenyl-1-(aminomethyl)ethene hydrochloride (PAME) is an olefinic substrate for dopamine beta-monooxygenase (DBM; EC 1.14.17.1) which inactivates the enzyme in an apparent mechanism-based manner. The present study further characterizes this reaction. The inactivation reaction yields kinact = 0.23 min-1 at pH 5.0 and 37 degrees C and is strictly dependent on reductant (ascorbate) and oxygen. The DBM/PAME substrate reaction (apparent kcat = 14 s-1), shown to be stimulated by fumarate, gives the corresponding epoxide as product, identified by derivatization with 4-(p-nitrobenzyl)pyridine. However, the lack of DBM inhibition by alpha-methylstyrene oxide, and the observation of identical PAME/DBM inactivation rates in the absence and presence of preformed enzymatic PAME epoxide, indicates that free epoxide is not the inactivating species. A structure-activity study revealed that 4-hydroxylation of PAME (to give 4-HOPAME) increases both kinact (0.81 min-1) and apparent kcat (56 s-1) values, while 3-hydroxylation (to give 3-HOPAME) greatly diminishes inactivation activity while retaining substrate activity (apparent kcat = 47 s-1). 4-Hydroxy-alpha-methylstyrene was found to be a DBM inhibitor (kinact = 0.53 min-1) with weak substrate activity (apparent kcat = 0.71 s-1), while 3-hydroxy-alpha-methylstyrene and alpha-(cyanomethyl) styrene were found not to exhibit detectable DBM substrate activity and only weak inhibitory activity. 3-Phenylpropargylamine hydrochloride showed no detectable DBM substrate activity but rapidly inactivated the enzyme. A new substrate activity for DBM was discovered, N-dealkylation of N-phenylethylenediamine and N-methyl-N-phenylethylenediamine, and the lack of O-dealkylation activity with phenyl 2-aminoethyl ether and 4-hydroxyphenyl 2-aminoethyl ether indicates that DBM N-dealkylation proceeds via initial one-electron abstraction from the benzylic nitrogen heteroatom. With this new substrate and inhibitor reactivity information in hand, along with the other known substrate reactions, a DBM oxygenation mechanism analogous to that for cytochrome P-450 is proposed.

The biological role of selenium is a subject of intense current interest, and the antioxidant activity of selenoenzymes is now known to be dependent upon redox cycling of selenium within their active sites. Exogenously supplied or metabolically generated organoselenium compounds, capable of propagating a selenium redox cycle, might therefore supplement natural cellular defenses against the oxidizing agents generated during metabolism. We now report evidence that selenium redox cycling can enhance the protective effects of organoselenium compounds against oxidant-induced DNA damage. Phenylaminoethyl selenides were found to protect plasmid DNA from peroxynitrite-mediated damage by scavenging this powerful cellular oxidant and forming phenylaminoethyl selenoxides as the sole selenium-containing products. The redox properties of these organoselenoxide compounds were investigated, and the first redox potentials of selenoxides in the literature are reported here. Rate constants were determined for the reactions of the selenoxides with cellular reductants such as glutathione (GSH). These kinetic data were then used in a MatLab simulation, which showed the feasibility of selenium redox cycling by GSH in the presence of the cellular oxidant, peroxynitrite. Experiments were then carried out in which peroxynitrite-mediated plasmid DNA nick formation in the presence or absence of organoselenium compounds and GSH was monitored. The results demonstrate that GSH-mediated redox cycling of selenium enhances the protective effects of phenylaminoethyl selenides against peroxynitrite-induced DNA damage.