VITAMINS

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Most vertebrates have two kinds of photoreceptor cells (rods and cones) in their retinas. In contrast, all the photoreceptor cells of nocturnal geckos are morphologically rods. Walls proposed an idea (Transmutation theory) that the gecko rods have evolved from cones of ancestral diurnal lizards. In order to elucidate whether the gecko rods have coneor rod-type visual pigments (photoreceptive retinal proteins), we studied two kinds of gecko visual pigments, gecko green and gecko blue. First, we isolated the cDNA clones of both the visual pigments and deduced their amino acid sequences. Gecko green and blue were highly similar in amino acid sequences to the chicken red- and green-sensitive cone visual pigments, respectively. Second, we extracted and purified the visual pigments from gecko retinas, and investigated their physiologically active form (meta II intermediate) with low temperature spectroscopy. The lifetimes of meta II intermediates of both the gecko visual pigments were much shorter than that of rhodopsin, and as short as those of cone visual pigments. From these results, we concluded that both the gecko visual pigments should be cone-type ones. The result strongly supports the Transmutation theory.

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S-Adenosylhomocysteine hydrolase (EC 3.3.1.1) catalyzes the reversible hydrolysis of S-adenosylhomocysteine to adenosine and homocysteine. The enzyme from rat liver is a tetramer consisting of identical subunits, each of which contains 1 mol of tightly bound NAD^+. The bound coenzyme is involved in the oxidation-reduction of nucleoside substrates, and is not released during catalysis. Spectroscopic studies showed that the mode of binding of NAD^+ to the enzyme was similar to that seen in many NAD^+-dependent dehydrogenases. In the middle of the sequence, S-adenosylhomocysteine hydrolase has the nucleotide-binding motif. Mutagenesis of key residues in this motif abolished or weakened NAD^+ binding .Thus, despite having a high affinity for NAD^+, the structure of the NAD^+ side of S-adenosylhomocysteine hydrolase is similar to those of dehydrogenases. The reaction with an active site-directed reagent, 5'-p-fluorosulfonylbenzoyladenosine and mutational analyses showed the occurrence of Cys78 in the S-adenosylhomocysteine-binding site. This residue is the only cysteine residue that is conserved through evolution.

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