The influence of biotin nutrition on carbon dioxide fixation and malic enzyme formation by Lactobacillus arabinosus.

Biotin has long been impliratcld in a number of carbon dioxide fixation reactions (see Lardy and Peanasky (1) for rcvicw). Two main proposals have been made to explain its mode of action: 1. The vitamin acts as a coenzyme in carbon dioxide fixation reactions. This view was based mainly on observations on the incorporation of radioactivity from C?“oz into compounds in equilibrium with certain substances, formed by fixation of COZ. It has been found that animals or microorganisms on a low biotin diet incorporated much less radioactivity from the same dose of C*Q into these substances than those receiving a normal level of the vitamin. Repletion esperiments with Lactobacillus arabinosus were particularly suggestive that biotin acts as a coenzgme, since suspensions of the vitamin-deficient organisms, after short periods of exposure to biotin, showed an increased fixation of C?Q into the cell structure (2-4). The function of biotin as a coenzymc has more recently been clarified in a number of reactions involving rarbon dioxide fixation (5-8)) and evidence for the nature of its mechanism of action in some of these has brcn presented (5, 6). 2. The second explanation for the action of biotin is that it plays a role in the synthesis of certain inducible enzymes or, perhaps, various other proteins. This view evolved from the observation that although the activity of “malic enzyme” was lower in preparations of liver from biotin-deficient turkeys than that in the tissues from normal birds, no significant amounts of biotin could be detected in purified preparations of this enzyme (9). A similar situation was cncountercd with an inducible malic enzyme (diphosphopyridinc nucleotide-linked) activity found in L. arabinosus. Evidence was presented that cells of L. arabinosus grown in a low biotin medium formed the inducible malic enzyme only upon supplementation with additional biotin; this was reported to occur in the absence of growth (10). The more recent reports on the role of biotin as a cornzymc of carboxylation (5-8) still leave unsolved it,s function in the re-