NADH and NADPH dehydrogenases from the blue-green alga,Aphanocapsa

Two different NAD(P)H dehydrogenases could be demonstrated in the blue-green alga, Aphanocapsa. Both function as quinone reductases using benzoquinone as electron acceptor. One, which was found in the soluble fraction, was NADH specific and showed high sensitivity to rotenone, thenoyltrifluoroacetone and o-phenanthroline. The second dehydrogenase was membrane-bound and used NADH as well as NADPH as substrates. Inhibition by rotenone and o-phenanthroline was less pronounced with the bound enzyme than with the soluble enzyme. Based on studies with NADH or NADPH, the membrane-bound enzyme apparently was associated with a low-temperature EPR signal at g=1.92 in the reduced state, indicative of an iron-sulfur center. The membrane-bound dehydrogenase was solubilized with Triton X-100 and partially purified. This preparation was used for studies of enzyme kinetics and acceptor specificity.Abbreviations DBMIB 2,5-dibromo methyl isopropylbenzoquinone - TTFA thenoyltrifluoroacetone - E _m midpoint redox potential