Kinetic comparison and regulation of the cascade of microsomal enzymes involved in renal arachidonate and endoperoxide metabolism.

Unilateral ureteral obstruction of rabbit kidney results, after 3 days, in an exaggerated release of prostaglandin E2 and thromboxane Az by the ex vivo perfused hydronephrotic kidney (HNK) but not by the surgically unaltered contralateral kidney (CLK). Both cyclooxygenase (PG endoperoxide synthetase) and thromboxane synthetase activities increased 20-40fold in cortical microsomes prepared from HNK compared with microsomes from CLK, but the highly active PG-endoperoxide-E2 isomerase rate did not change. In medullary microsomes, the high rate of cyclooxygenase activity was increased 2-%fold while the thromboxane synthetase activity increased 15-fold in HNK compared to CLK. In the 6-day HNK cortex, the cyclooxygenase (V,= 1000 pmol/mg of protein/min) was clearly the rate-limiting step, proceeding at only about one-fifth the rate of thromboxane synthetase ( L a x 5000 pmol/ mg/min) and only 1/40 the rate of the extremely rapid prostaglandin (PG) endoperoxide E2 isomerase (V,,, = 40-60,000 pmol/mg/min). Hydronephrotic cortical and medullary microsomes convert arachidonate into equivalent amounts of prostaglandin EZ and thromboxane Az. However, when the renal microsomes were incubated in the presence of reduced glutathione, the primary product was prostaglandin Ez and the ratio of prostaglandin E2 to thromboxane Az was about 10:l. Similarly, there was an 812: 1 ratio of PG endoperoxide EZ isomerase:thromboxane synthetase in the HNK. The ratio of PGEz to thromboxane AZ obtained in the renal venous effluent from a stimulated hydronephrotic kidney was also 8:1, suggesting that the intact tissue is dominated by the PG endoperoxide EZ isomerase and that reduced glutathione may be a key influence on the in vivo synthesis of prostaglandins in hydronephrosis.