A new sodium‐transport system energized by the decarboxylation of oxaloacetate

Oxaloacetate decarboxylase (EC 4.1 .1.3) from Klebsiella aerogenes [l] and methylmalonyl-CoA decarboxylase (EC 4.1 .1.41) from Micrococcus lactilyticus [2] have been shown to be avidin-sensitive and were therefore classified as biotin enzymes. It has further been assumed that these decarboxylases, like other biotin enzymes, perform a transfer reaction to synthesize the carboxy-biotin intermediate and that this intermediate is subsequently decomposed to CO2 and the free biotin enzyme [3]. This reaction mechanism has been established for oxaloacetate decarboxylase [4,5]. Interestingly, the second partial reaction was specifically dependent on the presence of NaC [S] and is therefore responsible for the unusual Na’ requirement of the decarboxylase in [ 11. This remarkable Na’ requirement and the localization of oxaloacetate decarboxylase in the membrane are reminiscent to the well-known Na’and K’-activated adenosintriphosphatase and have prompted us to hypothesize that an additional function of the decarboxylase could be the transport of Na’ through the membrane [4]. In this way the chemical energy liberated upon decarboxylation of oxaloacetate (AC N 7 kcal/mol) would be utilized to drive the transport of Na’ just as ATP energy is used to drive Na’ and K’ transport by the adenosintriphosphatase. The results described here agree with the above hypothesis. Inverted vesicles containing oxaloacetate decarboxylase were prepared from K. aerogenes and were found to accumulate Na’ in response to,the addition of oxaloacetate. Furthermore, oxaloacetate decarboxylation and sodium transport were simultanously inhibited by avidin treatment. 2. Materials and methods