Electron Acquisition System Constructed from an NAD-Independent D-Lactate Dehydrogenase and Cytochrome c₂ in Rhodopseudomonas palustris No. 7

Abstract

The activities of NAD-independent D- and L-lactate dehydrogenases (D-LDH, L-LDH) were detected in Rhodopseudomonas palustris No. 7 grown photoanaerobically on lactate. One of these enzymes, D-LDH, was purified as an electrophoretically homogeneous protein (M_r, about 235,000; subunit M_r about 57,000). The pI was 5.0. The optimum pH and temperature of the enzyme were pH 8.5 and 50°C, respectively. The Km of the enzyme for D-lactate was 0.8 mM. The enzyme had narrow substrate specificity (D-lactate and DL-2-hydroxybutyrate). The enzymatic activity was competitively inhibited by oxalate (Ki, 0.12 mM). The enzyme contained a FAD cofactor. Cytochrome c₂ was purified from strain No. 7 as an electrophoretically homogeneous protein. Its pI was 9.4. Cytochrome c₂ was reduced by incubating with D-LDH and D-lactate.