Discriminative effect of gamma-glutamyl transpeptidase inhibitors on metabolism of leukotriene C4 in peritoneal cells

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 C4, D4 and E4 (LTC4, LTD4 and LTE4) in response to stimulation with the calcium ionophore A23187 (1,2). The mechanism of enzymatic conversion of LTC4 to LTD4 is presumed to be via the action of gamma-glutamyl transpeptidase (gamma-GTPase, Figure 1) and has been demonstrated with purified enzymes from rat and porcine kidneys (3-6). We report that RPC contain gamma-GTPase-like activity that catalyzes the liberation of p-Nitroaniline (p-NA) from the chromophoric substrate gamma-glutamyl-p-nitroanilide (gamma-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 LTC4 to LTD4. Activity with gamma-GpNA is inhibited by D,L-gamma-glutamyl(o-carboxy)-phenylhydrazide (GOP) and serine-borate complex, (competitive inhibitors of kidney gamma-GTPase), and 6-diazo-5-oxo-L-norleucine (DON) and o-diazo-acetyl-L-serine (AZA), (irreversible inhibitors of kidney gamma-GTPase). In contrast, conversion of both endogenously-generated or exogenous LTC4 into LTD4 by RPC is inhibited only by serine-borate complex. These results suggest that RPC contain at least two distinct forms of gamma-GTPase; one capable of recognizing gamma-GpNA and susceptible to inhibition by all four compounds, and a second form utilizing also LTC4 as substrate, and is not inhibited by high concentrations of several "classic" gamma-GTPase inhibitors.