| During the initial minutes of anaerobiosis, ('14)C-labeled D-lactate, derived from the photosynthetic sugar phosphate pool, accumulated in the unicellular green alga, Chlamydomonas reinhardtii. The production of the D-isomer of lactate by algae is in contrast to plant and mammalian cells in which L-lactate is formed. After initial lactate formation, Chlamydomonas exhibits a mixed-acid type fermentation, thereby avoiding lactate accumulation and enabling the cells to tolerate extended periods of anaerobiosis.; A pyruvate reductase which catalyzes the formation of D-lactate in Chlamydomonas was partially purified and characterized. Lactate produced anaerobically was metabolized only when Chlamydomonas cells were returned to aerobic conditions, and reoxidation of the D-lactate was apparently catalyzed by a mitochondrial membrane-bound dehydrogenase, rather than by the soluble pyruvate reductase. Mutants of Chlamydomonas, deficient in mitochondrial respiration, were used to demonstrate that lactate metabolism was linked to the mitochondrial electron transport chain. In addition, the oxidation of glycolate, a structural analog of lactate, was also linked to mitochondrial electron transport in vivo.; The pyruvate reductase of Chlamydomonas also catalyzed the reduction of hydroxypyruvate and glyoxylate, both of which are intermediates of the oxidative photosynthetic carbon (C(,2)) cycle. Two other enzymes which catalyze the reduction of hydroxypyruvate and glyoxylate, but not pyruvate, were partially purified and characterized from Chlamydomonas reinhardtii. These enzymes, NAD:hydroxypyruvate reductase and NADPH:glyoxylate reductase, resembled the analogous C(,2) cycle enzymes from higher plants. |
