Cysteinyl
leukotrienes (LT) play an important role in the development of experimental
glomerulonephritis (GN). We have partially purified and characterized
LTC4 synthase, the
enzyme responsible for cysteinyl LT formation, from rat renal microsomes and have investigated this
enzyme activity in nephritic rats.
LTC4 formation, measured in vitro, was linear for > 10 min at 25 degrees C in the presence of 50 mM
serine borate (an inhibitor of
gamma-glutamyl transpeptidase), with Km values for
LTA4 and GSH of 56 microM and 8.5 mM, respectively.
Detergent solubilization and
anion-exchange chromatography of microsomal
proteins resulted in a 7-fold increase in
enzyme specific activity. Enzymatic and immunoblot analysis demonstrated that cytosolic and microsomal
glutathione S-transferase (GST) activities were distinct from
LTC4 synthase activity. Comparison of
LTC4 synthase activity in nephritic rats over 21 days revealed an initial increase over the first 24 h following injection of nephrotoxic sera, followed by a subsequent decline until day 7 and a gradual recovery by day 21. Inhibition of LT biosynthesis with
MK-0591 (10 mg kg-1 d-1) reduced GN-associated
proteinuria by 72% (P < 0.05). These results suggest a potential mechanism for enhanced cysteinyl LT formation in the development of experimental GN and further support their causal role in the etiology of this disease.