The reduction of vinyl side-chains of Mg-protoporphyrin IX monomethyl ester in vitro

Abstract

Portions of crude homogenates of etiolated wheat seedlings incubated with Mg-protoporphyrin IX and S- adenosyl- L- methionine and then added to other portions of the same crude homogenates that were pretreated with [1- 3H]ethanol and yeast alcohol dehydrogenase provided, after a short reaction period, 3H-labeled Mg-protoporphyrin IX monomethyl ester. The 3H-labeled Mg-protoporphyrin IX monomethyl ester thus obtained was shown to contain the 3H in one reduced (to ethyl) vinyl side-chain. Subsequently, 3H-labeled Mg-monoethyl-(monodivinyl)-protoporphyrin IX monomethyl ester was obtained when Mg-protoporphyrin IX monomethyl ester and [ 3H]NADH were added to dialyzed crude homogenates of etiolated wheat seedlings. Insignificant amounts of 3H were incorporated into poprhyrin substrates when Mg-2,4-divinylpheoporphyrin a 5 or [ 3H]NADPH were substituted in reaction mixtures for Mg-protoporphyrin IX monomethyl ester or [ 3H]NADPH, respectively. The results of these and further experiments suggest that an NADPH-dependent enzyme in the crude homogenates of etiolated wheat seedlings was capable of catalyzing the reduction to ethyl of one vinyl side-chain of Mg-protoporphyrin IX monomethyl ester. These findings suggest that the 4-vinyl side-chain reductive reaction likely occurs after the biosynthesis IX monomethyl ester, but before isocyclic ring formation in the pathway to chlorophyll a.