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Previous studies with maleylacetone cis-trans-isomerase suggest that the coenzyme, glutathione (GSH), may act as a nucleophile in the enzyme-catalyzed cis-trans isomerization. In the absence of enzyme, reaction of the coenzyme with maleylacetone or its trans isomer, fumarylacetone, has been found to lead to an adduct, S-2-(4,6-dioxoheptanoic acidjglutathione. The same diastereomer of the adduct is formed from either geometric isomer, suggesting that the reaction of GSH with maleylacetone proceeds by a slow catalyzed cis-trans isomerization followed by a rapid reaction of GSH conjugate addition to the trans isomer. This mechanism is confirmed by an isotope dilution experiment and a kinetic method involving repetitive addition of fractional equivalents of GSH to a solution of maleylacetone. The intermediate formed during GSH-catalyzed cis-trans isomerization is suggested to be the dienediol formed by sulfur attack at C-2, and this intermediate undergoes bond rotation and elimination before protonation at C-3 can take place. By analogy and additional evidence, a similar mechanism and intermediate are suggested for the enzyme-coenzyme-substrate reaction.Aromatic amino acids such as tyrosine and phenylalanine are metabolized in mammalian liver to a common intermediate, homogentisic acid (2,5-dihydroxyphenylacetic acid). Oxidative cleavage of the aromatic ring yields maleylacetoacetate (la), which in turn undergoes enzyme-catalyzed cistrans isomerization to fumarylacetoacetate (lia, eq 1 ).2 Enzymes capable of carrying out these same transformations are also found in bacteria such as Vibrio 01.3 The focus of attention in this paper is concerned with the mechanism of the catalyzed cis-trans isomerization reaction.The mechanistic investigation is simplified to some degree by the finding that maleylacetone (lb) is also rapidly isomerized by both the liver and bacterial cis-trans-isomerase. Action

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Abstract— On irradiation of all‐trans 5,6‐dihydroretinal (I), two opsin‐active isomers are formed. Both these isomers (believed to be the 11‐cis and 9‐cis isomers) individually couple with cattle opsin to form complexes which have maximal absorption at 465 nm. These complexes satisfy all the established criteria characteristic of synthetic visual pigments.