Vascular and endothelial actions of inhibitors of substance P amidation

Formation of mature active neuropeptides such as substance P (SP) from their glycine extended precursors entails alpha-amidation of peptide precursors by the sequential enzymatic action of peptidylglycine alpha-monooxygenase (PAM) and peptidylamidoglycolate lyase (PGL). We reported that these two enzymes that can produce mature active neuropeptides are present in cultured bovine aortic endothelial cells (BAECs). We hypothesize that alpha-amidation of peptides occurs in endothelial cells and that these peptides are critically involved in the overall regulation of cardiovascular function. In this study, this hypothesis was tested using specific amidation inhibitors to determine their effects on the actions of SP and its glycine-extended precursor (SP-Gly). We have found that SP and SP-Gly are equipotent in stimulating nitric oxide (NO) release by BAECs. At 10(-5) M, the specific inhibitors of PAM (4-phenyl-3-butenoic acid; PBA) and PGL (5-acetamido-2,4-diketo-6-phenyl-hexanoic acid and its methyl ester) reduced NO basal release by 40, 34, and 45%, respectively. They also reduced the production of NO induced by SP-Gly by 63, 68, and 69%, respectively, but had no effect on NO production in response to either SP or acetylcholine. SP and SP-Gly also were equipotent in relaxing rat aortic segments. The vasorelaxation to SP-Gly was endothelium dependent and inhibited by the NOS antagonist L-nitroarginine methyl ester (L-NAME), but it was not affected by inhibition of prostaglandin synthesis. Inhibitors of both PAM and PGL significantly reduced the vasorelaxing actions of SP-Gly, whereas responses to SP were not affected. A cumulative infusion of PBA into the femoral artery of rabbits, at final concentrations of 2.4, 24, and 240 microM for 20 min each, increased the vascular resistance (VR), indicating the tonic production of vasodilating amidated peptide(s). This effect was maximum at 60 min after infusion (20.5 +/- 4.7 vs. 8.2 +/- 0.7 mm Hg/ml/min; p < 0.05). These results suggest that endothelial cells can produce mature SP from its SP-Gly precursor and that a product of peptide alpha-amidation tonically stimulates endothelial cell NO release to control vascular tone.

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.

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.