Rat lens prostaglandin generation proceeds by the non-enzymatic degradation of PGH2 endoperoxide.

Experiments were performed using [1-14C]PGH2 endoperoxide as substrate to assess which, if any, of the PGH2 utilizing enzymes were present in rat lens microsomes. After a 15 min incubation, prostaglandins (PGs) were extracted, separated by thin-layer chromatography and labeled PGs located by autoradiography. The bands were cut and counted and product formation expressed as a percentage of the total recovered label. Prostaglandin PGE2 formation accounted for 24% of the label and was not increased (22%) when 1 mM reduced glutathione (GSH), the obligate cofactor of PGH2----PGE2 isomerase, was added. However, the addition of the GSH significantly increased PGF2 alpha generation from 25% to 37%. Pretreatment of the microsomes with p-hydroxymercuribenzoate, a sulfhydryl-blocking agent and an inhibitor of PGH2----PGE2 isomerase, did not limit lens microsomal PGE2 formation (24% vs. 39%) but dramatically inhibited PGF2 alpha production (25% vs. 6%). Boiling the microsomes did not alter PGF2 alpha or PGE2 generation suggesting that these PGs were non-enzymatically produced. No thromboxane B2 or 6-keto-PGF1 alpha (the stable breakdown products of thromboxane A2 and prostacyclin, respectively) were found indicating that their respective synthetases were absent in the lens microsomes. These data suggest that the lens is a unique tissue, generating the primary PGs in the absence of all known PGH2 endoperoxide-utilizing enzymes. It appears that the PGH2 formed by lens cyclo-oxygenase spontaneously breaks down in the lens microsomes into PGE2, PGD2 and PGF2 alpha.