Discriminative effect of γ-glutamyl transpeptidase inhibitors on metabolism of leukotriene C 4 in peritoneal cells

Abstract

Leukotrienes are naturally-occurring metabolites of arachidonic acid that are formed via the 5-lipoxygenase pathway in several tissues. Rat peritoneal cells (RPC) can produce leukotrienes C 4, D 4 and E 4 (LTC 4, LTD 4 and LTE 4) in response to stimulation with the calcium ionophore A23187 (1, 2). The mechanism of enzymatic conversion of LTC 4 to LTD 4 is presumed to be via the action of γ-glutamyl transpeptidase (γ-GTPase, Figure 1) and has been demonstrated with purified enzymes from rat and porcine kidneys (3–6). We report that RPC contain γ-GTPase-like activity that catalyzes the liberation of p-Nitroaniline (p-NA) from the chromophoric substrate γ-glutamyl-p-nitroanilide (γ-GpNA) in the presence of the acceptor molecules glycylglycine and L-cysteine. Furthermore, we demonstrate that under similar conditions, this preparation catalyzes the conversion of LTC 4 to LTD 4. Activity with γ-GpNA is inhibited by D, L-γ-glutamyl (o-carboxy)-phenylhydrazide (GOP) and serine-borate complex, (competitive inhibitors of kidney γ-GTPase), and 6-diazo-5-oxo-L-norleucine (DON) and o-diazo-acetyl-L-serine (AZA), (irreversible inhibitors of kidney γ-GTPase). In contrast, conversion of both endogenously-generated or exogenous LTC 4 into LTD 4 by RPC is inhibited only by serine-borate complex. These results suggest that RPC contain at least two distinct forms of γ-GTPase; one capable of recognizing γ-GpNA and susceptible to inhibition by all four compounds, and a second form utilizing also LTC 4 as substrate, and is not inhibited by high concentration of several “classic” γ-GTPase inhibitors.