Effects of metal ions on 15-hydroxy prostaglandin dehydrogenase activity in rabbit kidney cortex

Cyclooxygenase metabolites in the kidney

In the mammalian kidney, prostaglandins (PGs) are important mediators of physiologic processes, including modulation of vascular tone and salt and water. PGs arise from enzymatic metabolism of free arachidonic acid (AA), which is cleaved from membrane phospholipids by phospholipase A2 activity. The cyclooxygenase (COX) enzyme system is a major pathway for metabolism of AA in the kidney. COX are the enzymes responsible for the initial conversion of AA to PGG2 and subsequently to PGH2, which serves as the precursor for subsequent metabolism by PG and thromboxane synthases. In addition to high levels of expression of the "constitutive" rate-limiting enzyme responsible for prostanoid production, COX-1, the "inducible" isoform of cyclooxygenase, COX-2, is also constitutively expressed in the kidney and is highly regulated in response to alterations in intravascular volume. PGs and thromboxane A2 exert their biological functions predominantly through activation of specific 7-transmembrane G-protein-coupled receptors. COX metabolites have been shown to exert important physiologic functions in maintenance of renal blood flow, mediation of renin release and regulation of sodium excretion. In addition to physiologic regulation of prostanoid production in the kidney, increases in prostanoid production are also seen in a variety of inflammatory renal injuries, and COX metabolites may serve as mediators of inflammatory injury in renal disease.

Prostaglandin catabolizing enzymes

The primary catabolic pathway of prostaglandins and related eicosanoids is initiated by the oxidation of 15(S)-hydroxyl group catalyzed by NAD+-dependent 15-hydroxyprostaglandin dehydrogenase (15-PGDH) followed by the reduction of delta13 double bond catalyzed by NADPH/NADH dependent delta13-15-ketoprostaglandin reductase (13-PGR). 13-PGR was also found to exhibit NADP+-dependent leukotriene B4 12-hydroxydehydrogenase (12-LTB4DH) activity. These enzymes are considered to be the key enzymes responsible for biological inactivation of prostaglandins and related eicosanoids. A separate catabolic pathway of thromboxane involves the oxidation of thromboxane B2 (TXB2) at C-11 catalyzed by NAD+-dependent 11-hydroxythromboxane B2 dehydrogenase (11-TXB2DH). The product of this reaction, 11-dehydro-TXB2, has been considered to be a more reliable quantitative index of thromboxane formation in the circulation. Recent biochemical and molecular biological studies have revealed interesting catalytic properties, structure, and activity relationship, and regulation of gene expression of these three enzymes. Future investigation may shed more light on the roles of these enzymes in health and diseases.

Effects of Fe(2+), Zn(2+), Cu(2+) and Se(4+) on the synthesis and catabolism of prostaglandins in rabbit gastric antral mucosa

Effects of Fe(2+), Zn(2+), Cu(2+) and Se(4+) on the synthesis and catabolism of prostaglandins (PGs) in rabbit gastric antral mucosa were examined. Fe(2+) inhibited the cyclooxygenase activity in the microsomal fraction. Zn(2+) suppressed the endoperoxide E(2) isomerase activity in the microsomal fraction and the 15-hydroxy PG dehydrogenase (PGDH) activity in the cytosolic fraction. Cu(2+) stimulated the cyclooxygenase activity, inhibited the PGDH activity and possibly induced non-enzymatic reduction of PGG(2) or PGH(2) to PGF(2 alpha) Se(4+) possibly induced the non-enzymatic reduction of PGG(2) or PGH(2) to PGF(2 alpha), as well as Cu(2+). These results suggest that Fe(2+), Zn(2+), Cu(2+) and Se(4+) can be modulators of the gastric antral mucosal PG levels by affecting the PG synthesizing enzymes, PG catabolizing enzyme and/or non-enzymatic reduction of PGG or PGH to PGF.

Inhibition of prostaglandin delta 13 reductase activity in rabbit kidney cortex by glutathione disulfide

t-Butyl hydroperoxide and H2O2-Fe(2+)-EDTA-glutathione system which produces hydroxyl radicals did not affect the 15-hydroxy prostaglandin dehydrogenase activity in rabbit kidney cortex. On the other hand, H2O2-Fe(2+)-EDTA-glutathione system inhibited the prostaglandin delta 13 reductase activity. Mannitol, a scavenger of hydroxyl radicals, had no effect on the inhibitory action of this system, indicating that the effect of H2O2-Fe(2+)-EDTA-glutathione system on the prostaglandin delta 13 reductase may not be due to produced hydroxyl radicals. As a result of further investigation, it was shown that glutathione disulfide, which is synthesized concomitantly with hydroxyl radicals from H2O2-Fe(2+)-EDTA-glutathione, inhibited the prostaglandin delta 13 reductase activity. These results suggest that hydroperoxides and hydroxyl radicals may not be likely candidates for the modulator of the catabolism of prostaglandins in the kidney cortex, and that glutathione disulfide has the potential to modulate the prostaglandin catabolism by affecting the prostaglandin delta 13 reductase activity.