Transport and metabolism of vitamin B6 in lactic acid bacteria

Streptococcus faecalis 8043 concentrates extracellular [3H]pyridoxal or [3H]pyridoxamine primarily as the corresponding 5'-phosphates. Accumulation of pyridoxamine requires an exogenous energy source and is inhibited by glycolysis inhibitors. A membrane potential is not required for transport of pyridoxamine, and an artificially generated potential does not drive uptake in this organism. Based on this and other evidence, it is concluded that S. faecalis accumulates pyridoxamine by facilitated diffusion in conjunction with trapping by pyridoxal kinase. Pyridoxamine-P is not concentrated, but equilibrates with that provided externally. Lactobacillus casei 7469 concentrates radioactivity only from pyridoxal, which appears internally as pyridoxal-P, suggesting that it too absorbs the vitamin by facilitated diffusion plus trapping. The specificity of the growth requirement of S. faecalis and L. casei for vitamin B6 parallels the specificity of the transport systems for this vitamin in these organisms. Lactobacillus delbrueckii 7469, however, which specifically requires pyridoxamine-P or pyridoxal-P for growth, accumulates both these compounds and pyridoxine-P from the medium, apparently by active transport, but not pyridoxine, pyridoxamine, or pyridoxal. While pyridoxal-P and pyridoxamine-P are interconvertible in this organism, pyridoxine-P is not further metabolized, thus accounting for the specificity of the growth requirement. These and previous results show (a) that different organisms may employ quite different transport machinery in utilization of a given external nutrient, and (b) that the specificity of the growth requirement for a given form of a vitamin frequently arises from the specificity of transport, but that internal metabolism of the compounds also plays a significant role in some organisms.