Studies on the substrate specificity and inhibition of prostaglandin biosynthesis with methyl branched isomers of eicosa-8, 11, 14-trienoic acid

Seven radioactive methyl branched isomers of eicosa-8, 11, 14-trienoic acid, in which a methyl branch was located at carbon 2, 5, 10, 13, 17, 18, or 19, were used as substrates to study their enzymatic conversion into homologues of prostaglandin E1 by microsomes from bovine vesicular glands. The products of this reaction were partially characterized. When the methyl branch was moved towards position 13, the site of stereospecific hydrogen removal by prostaglandin synthetase, the rate of prostaglandin formation declined rapidly. When a methyl branch was at positions 10, 13, 17, 18, or 19, the rates of prostaglandin formation were less than 25% of that from eicosa-8, 11, 14-trienoic acid. However, when a methyl branch was at positions 2 or 5, these isomers were converted to prostaglandins at least 50% as rapidly as eicosa-8, 11, 14-trienoic was converted to prostaglandins. The apparent Km and Vmax values for eicosa-8, 11, 14-trienoic acid were respectively 224 μM and 4.62 nmoles/min/mg protein. Kinetic data suggest that the enzyme converted eicosa-8, 11, 14-trienoic acid to prostaglandins 3.6 times faster than the 5-methyl branched isomer, although the enzyme bound the branched isomer twice as firmly as eicosa-8, 11, 14-trienoic acid. The concentration-dependent inhibition of prostaglandin formation from eicosa-8, 11, 14-trienoic acid by the 13-methyl branched isomer was also demonstrated.