A new facile trinitrophenylated substrate for peptide alpha-amidation and its use to characterize PAM activity in chromaffin granules

Catalytically active peptidylglycine alpha-amidating monooxygenase in the media of androgen-independent prostate cancer cell lines

Peptidylglycine alpha-amidating monooxygenase (PAM) converts inactive terminal-glycine prohormones into their activated alpha-amidated forms. PAM is thought to play a role in the development of antiandrogen drug resistance in prostate cancer (CaP) through PAMactivated autocrine growth. On the basis of the previous finding that many lung cancer cell lines excrete PAM into their culture media, this study investigates PAM levels in media collected from human CaP cell line cultures. Androgen-independent DU145 and PC-3 prostate cancer cell lines exhibited readily detectable levels of PAM activity in extracts and media, whereas the androgen-dependent LNCaP cell line showed little or no activity. Because of the much larger volume of media versus cell extracts, more than 90% of the total PAM activity was located in the media for both the PC-3 and DU145 cell lines, providing a readily accessible source of CaP PAM. A simple, scalable method to obtain PAM from the culture media of androgen-independent human prostate cancer cell lines is described in this article. This approach provides a much easier means of collecting CaP-derived PAM than previously described cell fractionation procedures and should facilitate the investigations of the role and targeting of PAM in hormone-independent CaP.

Catalysis, stereochemistry, and inhibition of ureidoglycolate lyase

Ureidoglycolate lyase (UGL, EC 4.3.2.3) catalyzes the breakdown of ureidoglycolate to glyoxylate and urea, which is the final step in the catabolic pathway leading from purines to urea. Although the sequence of enzymatic steps was worked out nearly 40 years ago, the stereochemistry of the uric acid degradation pathway and the catalytic properties of UGL have remained very poorly described. We now report the first direct investigation of the absolute stereochemistry of UGL catalysis. Using chiral chromatographic analyses with substrate enantiomers, we demonstrate that UGL catalysis is stereospecific for substrates with the (S)-hydroxyglycine configuration. The first potent competitive inhibitors for UGL are reported here. These inhibitors are compounds which contain a 2,4-dioxocarboxylate moiety, designed to mimic transient species produced during lyase catalysis. The most potent inhibitor, 2,4-dioxo-4-phenylbutanoic acid, exhibits a KI value of 2.2 nM and is therefore among the most potent competitive inhibitors ever reported for a lyase enzyme. New synthetic alternate substrates for UGL, which are acyl-alpha-hydroxyglycine compounds, are described. Based on these alternate substrates, we introduce the first assay method for monitoring UGL activity directly. Finally, we report the first putative primary nucleotide and derived peptide sequence for UGL. This sequence exhibits a high level of similarity to the fumarylacetoacetate hydrolase family of proteins. Close mechanistic similarities can be visualized between the chemistries of ureidoglycolate lyase and fumarylacetoacetate hydrolase catalysis.

Expression and characterization of human bifunctional peptidylglycine alpha-amidating monooxygenase

We report the purification and characterization of human bifunctional peptidylglycine alpha-amidating monooxygenase (the bifunctional PAM) expressed in Chinese hamster ovary cells. PAM is in charge of the formation of the C-terminal amides of biologically active peptides. The bifunctional PAM possesses two catalytic domains in a single polypeptide, peptidylglycine alpha-hydroxylating monooxygenase (PHM, EC 1.14.17.3) and peptidylamidoglycolate lyase (PAL, EC 4.3.2.5). By introducing a stop codon at 835 Glu, we were able to eliminate the membrane-spanning domain in the C-terminal region and succeeded in purifying a soluble form of bifunctional PAM that was secreted into the medium. Through a three-step purification procedure, we obtained 0.3mg of the purified PAM, which showed a single band at 91 kDa on SDS-PAGE, from 1L of monolayer culture medium. Metals contained in the purified PAM were analyzed and chemical modifications were performed to gain insight into the mechanism of the PAL reaction. Inductively coupled plasma detected 0.62 mol of Zn(2+) and 1.25 mol of Cu(2+) per mol of bifunctional PAM. Further, the addition of 1mM EDTA reduced the PAL activity by about 50%, but the decreased activity was recovered by the addition of an excess amount of Zn(2+). In a series of chemical modifications, phenylglyoxal almost completely eliminated the PAL activity and diethyl pyrocarbonate suppressed activity by more than 70%. These findings implied that Arg and His residues might play crucial roles during catalysis.

Expression and characterization of frog peptidylglycine alpha-hydroxylating monooxygenase

We report here a recombinant Chinese hamster ovary cell system,which is able to stably express frog peptidylglycine alpha-hydroxylating monooxygenase (PHM, EC 1.14.17.3), the first enzyme responsible for the formation of peptide C-terminal amide. This system excreted PHM mostly into the medium and almost no PHM activity was detected in the cell lysate. Three differentiation inducers were examined to determine whether or not they would enhance the PHM expression. Addition of 4mM sodium butyrate into the medium increased the expression of PHM activity about 4-fold at 48 h after addition. Increases of about 2-fold were observed in the cases of sodium propionate or N,N(')-hexamethylene-bis-acetamide. Through a three-step purification procedure, we obtained 5mg purified PHM, which showed a single band at 40 kDa on SDS-PAGE, from 2-L of conventional monolayer culture medium. The reactions with three synthetic substrates, D-Tyr-Val-Gly, N-trinitrophenyl-D-Tyr-Val-Gly (TNPYVG), and hippuric acid (HA), were characterized. Of these, TNPYVG was the most active substrate. The pH optima for TNPYVG and HA were pH 5-6, while that for D-Tyr-Val-Gly was pH 7.5. There is a possibility that the substrate N-terminal structure may affect the interaction between the substrate and the enzyme catalytic site.

Amidative peptide processing and vascular function

Substance P (SP), an amidated peptide present in many sensory nerves, is known to affect cardiovascular function, and exogenously supplied SP has been shown to activate nitric oxide synthase (NOS) in endothelial cells. We now report that SP-Gly, the glycine-extended biosynthetic precursor of SP (which is enzymatically processed to the mature amidated SP), causes relaxation of rat aortic strips with an efficacy and potency comparable to that of SP itself. Pretreatment of the aortic strips with 4-phenyl-3-butenoic acid (PBA), an irreversible amidating enzyme inactivator, results in marked inhibition of the vasodilation activity induced by SP-Gly but not of that induced by SP itself. Isolated endothelial cell basal NOS activity is also decreased by pretreatment with PBA, with no evidence of cell death or direct action of PBA on NOS activity. Both bifunctional and monofunctional forms of amidating enzymes are present in endothelial cells, as evidenced by affinity chromatography and Western blot analysis. These results provide evidence for a link between amidative peptide processing, NOS activation in endothelial cells, and vasodilation and suggest that a product of amidative processing provides intrinsic basal activation of NOS in endothelial cells.

Reaction versus subsite stereospecificity of peptidylglycine alpha-monooxygenase and peptidylamidoglycolate lyase, the two enzymes involved in peptide amidation

Carboxyl-terminal amidation, a required post-translational modification for the bioactivation of many neuropeptides, entails sequential enzymatic action by peptidylglycine alpha-monooxygenase (PAM, EC 1.14.17.3) and peptidylamidoglycolate lyase (PGL, EC 4.3.2.5). The monooxygenase, PAM, first catalyzes conversion of a glycine-extended pro-peptide to the corresponding alpha-hydroxyglycine derivative, and the lyase, PGL, then catalyzes breakdown of this alpha-hydroxyglycine derivative to the amidated peptide plus glyoxylate. We have previously established that PAM and PGL exhibit tandem reaction stereospecificities, with PAM producing, and PGL being reactive toward, only alpha-hydroxyglycine derivatives of absolute configuration (S). We now demonstrate that PAM and PGL exhibit dramatically different subsite stereospecificities toward the residue at the penultimate position (the P2 residue) in both substrates and inhibitors. Incubation of Ac-L-Phe-Gly, Ac-L-Phe-L-Phe-Gly, or (S)-O-Ac-mandelyl-Gly with PAM results in complete conversion of these substrates to the corresponding alpha-hydroxylated products, whereas the corresponding X-D-Phe-Gly compounds undergo conversions of < 1%. The KI of Ac-D-Phe-Gly is at least 700-fold higher than that of Ac-L-Phe-Gly, and the same pattern holds for other substrate stereoisomers. This S2 subsite stereospecificity of PAM also holds for competitive inhibitors; thus, the KI of 45 microM for Ac-L-Phe-OCH2CO2H increases to 2,247 microM for the -D-Phe- enantiomer. In contrast, incubation of PGL with Ac-L-Phe-alpha-hydroxy-Gly, Ac-D-Phe-alpha-hydroxy-Gly, (S)-O-Ac-mandelyl-alpha-hydroxy-Gly, or (R)-O-Ac-mandelyl-alpha-hydroxy-Gly results in facile enzymatic conversion of each of these compounds to their corresponding amide products. The simultaneous expression of high reaction stereospecificity and low S2 subsite stereospecificity in the course of PGL catalysis was illustrated by a series of experiments in which enzymatic conversion of the diastereomers of Ac-L-Phe-alpha-hydroxy-Gly and Ac-D-Phe-alpha-hydroxy-Gly was monitored directly by HPLC. Kinetic parameters were determined for both substrates and potent competitive inhibitors of PGL, and the results confirm that, in sharp contrast to PAM, the configuration of the chiral moiety at the P2 position has virtually no effect on binding or catalysis. These results illustrate a case where catalytic domains, which must function sequentially (and with tandem reaction stereochemistry) in a given metabolic process, nevertheless exhibit sharply contrasting subsite stereospecificities toward the binding of substrates and inhibitors.

NN-dimethyl-1,4-phenylenediamine as an alternative reductant for peptidylglycine alpha-amidating mono-oxygenase catalysis

C-terminal alpha-amidation is a structural feature essential to the biological activity of many peptide hormones. Peptidylglycine alpha-amidating mono-oxygenase (PAM; EC 1.14.17.3) catalyses conversion of glycine-extended peptide hormone precursors into their corresponding alpha-hydroxyglycine derivatives. This reaction is the first step in the C-terminal amidation process. We report here that in the presence of molecular O2, copper and PAM substrate, NN-dimethyl-1,4-phenylenediamine (DMPD) serves as the requisite electron donor for the mono-oxygenase, being oxidized in the process to a stable and highly chromophoric cation radical. By monitoring the rate of increase in absorbance at 515 nm, PAM activity can be easily followed. This provides a spectrophotometric assay for PAM, which represents the first continuous assay reported for this enzyme. DMPD-supported PAM-catalysed mono-oxygenation exhibits normal Michaelis-Menten kinetic behaviour. Steady-state kinetic studies established that both the ascorbate-supported and DMPD-supported PAM reactions exhibit apparent 'Ping Pong' kinetics. In addition, both electron donors give rise to similar pH profiles and identical inhibition patterns towards known competitive inhibitors of PAM. The stoichiometry between formation of the DMPD cation radical and the alpha-hydroxyglycine PAM product was determined to be 2:1, the value expected for a monooxygenase-catalysed reaction. The optimum pH for the DMPD-supported continuous PAM assay was found to be about 5.5. The major advantage of this assay over all previously reported methods is that it is continuous; thus accurate initial rates are easily obtained. Moreover, unlike previous assay methods, 125I-labelled or chromophorically modified substrates are not required. Kinetic parameters for a broad range of PAM substrates and inhibitors have been successfully obtained using this assay.

Peptide amidating enzymes are present in cultured endothelial cells

Carboxy-terminal amidation is a prevalent post-translational modification necessary for the bioactivity of many peptides. We now report that the two enzymes essential for amidation, peptidylglycine alpha-monooxygenase (PAM) and peptidylamidoglycolate lyase (PGL), are present in both the cytosol and membrane fractions of cultured bovine aortic endothelial cells. Endothelial PAM exhibits ascorbate-dependent turnover and is inactivated by the mechanism-based inactivator, 4-phenyl-3-butenoic acid (PBA), whereas PGL activity is independent of ascorbate and is not affected by PBA. These enzymological characteristics correspond to those of amidating enzymes from other tissues. These results suggest a heretofore unrecognized role for alpha-amidated peptides in cardiovascular function.

A colorimetric assay for measuring peptidylglycine alpha-amidating monooxygenase using high-performance liquid chromatography

In many peptide hormones and neuropeptides, the carboxy-terminal alpha-amide structure is essential in eliciting biological activity. In the present study, a rapid and sensitive assay method for the determination of peptidylglycine alpha-amidating monooxygenase (PAM) activity has been reported. This method is based on the monitoring of the absorption at 460 nm of 4-dimethylaminoazobenzene-4'-sulfonyl-Gly-L-Phe-NH2 (Dabsyl-Gly-Phe-NH2), enzymatically formed from the substrate 4-dimethylaminoazobenzene-4'-sulfonyl-Gly-L-Phe-Gly, after separation by high-performance liquid chromatography (HPLC) using a C-18 reversed-phase column by isocratic elution. This method is sensitive enough to measure Dabsyl-Gly-Phe-NH2 at concentrations as low as 1 pmol and yield highly reproducible results and requires less than 5 min per sample for separation and quantitation. The concentrations of copper and ascorbic acid required for maximal enzyme activity were 1 microM and 2 mM, respectively. The pH optimum for PAM activity was 5.0 to 5.5. The Km and Vmax values were respectively 3.5 microM and 100 pmol/micrograms/h with the use of enzyme extract obtained from bovine pituitary. By using this method, PAM activity could be readily detected in a single rat saliva. The sensitivity of this assay method will also aid in the effort to examine the regulation of in vivo PAM activity.

Determination of peptidylglycine alpha-amidating monooxygenase activity in human serum by thin-layer chromatography

We developed a simple assay system for the quantitative evaluation of peptidylglycine alpha-amidating monooxygenase activity using as substrate a 125I-labeled synthetic tripeptide, 125I-D-Tyr-Val-Gly, thin-layer chromatography, and a radiochromatoscanner. The basic principle of this method is that thin-layer chromatography separates the reaction product, 125I-D-Tyr-Val-NH2, from the substrate in an assay mixture. The 125I activities of both substrate and product separated from each other on a thin-layer chromatography plate were quantified with a radiochromatoscanner and the rate of conversion of the substrate to the product was calculated from their counts. Human serum was used as an enzyme source and the values of alpha-amidation activity obtained by our method under optimal conditions were almost identical to those of the published method using ion-exchange chromatography (sulphopropyl-Sephadex C-50 column) and a gamma-counter. Our method makes it possible to estimate the 10-pmol level of the product using 10 microliters of human serum and to assay a large number of samples rapidly and easily. It is therefore thought to be very useful for screening various tissues for alpha-amidation activity.

Kinetic analyses of peptidylglycine alpha-amidating monooxygenase from pancreatic islets

Peptidylglycine alpha-amidating monooxygenase (PAM) plays an important role in the post-translational processing of bioactive neuropeptides by participating in C-terminal amidation. We have examined PAM activity in the pancreatic islets of the anglerfish (AF), Lophius americanus. It was previously demonstrated that the cofactor requirements and pH optimum for the fish PAM are essentially identical to PAM obtained from other tissues and species. The present study was performed to examine the enzymatic characteristics of the fish islet PAM in more detail. One of the questions addressed was the suitability of the AF islet neuropeptide Y-like peptide, aPY-Gly, as a substrate for the islet PAM. Partially purified PAM from AF islet secretory granules was incubated with [125I] aPY-Gly and the resulting products were analyzed by HPLC. The islet PAM readily mediated the formation of aPY-amide from aPY-Gly. PAM purified from bovine adrenal chromaffin granules also catalyzed the amidation of [125I] aPY-Gly. The kinetic parameters of the islet PAM were examined using trinitrophenylated-D-Tyr-Val-Gly (TNP-D-YVG) and 4-nitrohippuric acid (4-NHA). The Km of the islet PAM was 25 +/- 5 microM for TNP-D-YVG and 3.4 +/- 1 mM for 4-NHA. The competitive inhibitor of mammalian PAM activity, 4-methoxybenzoxyacetic acid, proved to be a potent inhibitor of the islet PAM as well, with an apparent KI of 0.06 mM. These results demonstrate that the AF islet PAM exhibits substrate compatibility, kinetic parameters, and inhibitor susceptibility quite similar to the characteristics of PAM from other tissues and species.

Functional and structural characterization of peptidylamidoglycolate lyase, the enzyme catalyzing the second step in peptide amidation

Carboxy-terminal amidation is a prevalent posttranslational modification necessary for the bioactivity of many neurohormonal peptides. We recently reported that in addition to peptidylglycine alpha-monooxygenase (PAM), a second enzyme, which we now call peptidylamidoglycolate lyase (PGL), functions in the enzymatic formation of amides [Katopodis et al. (1990) Biochemistry 29, 4551]. The monooxygenase first catalyzes formation of the alpha-hydroxyglycine derivative of the glycine-extended precursor, and the lyase subsequently catalyzes breakdown of the PAM product to the amidated peptide and glyoxylate. We report here the first primary sequence data for PGL, which establish that it is part of the putative protein precursor which also contains PAM. We also show that PAM and PGL activities are colocalized in the secretory granular fraction of neurointermediate pituitary as would be expected for enzymes sharing the same precursor. Time course studies of the amidation reaction using purified soluble pituitary PAM and PGL indicate that both enzymes are essential for enzymatic amidation. Finally, PGL has no effect on the substrate or inhibition kinetics of PAM, and purified pituitary PAM has an acidic pH optimum consistent with its known localization in secretory granules.

A novel enzyme from bovine neurointermediate pituitary catalyzes dealkylation of alpha-hydroxyglycine derivatives, thereby functioning sequentially with peptidylglycine alpha-amidating monooxygenase in peptide amidation

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.