Cell permeability and decarboxylation of α-keto acids by intact yeast

Abstract

Straight-chain α-keto acids containing up to at least eight carbon atoms are decarboxylated by baker's and brewer's yeast preparations in which the cell membrane has been destroyed by various methods. The decarboxylation order is similar to that occurring when yeast carboxylase is used, the velocity being the slower, the longer the carbon chain of the keto acid. Straight-chain α-keto acids containing up to eight carbon atoms are decarboxylated at low substrate concentrations by intact baker's or brewer's yeast, the speed of the process increasing with the chain length of the acid. With all the keto acids investigated, a maximum decarboxylating activity occurred at a certain concentration. When the substrate concentration exceeded this level, the decarboxylation showed a sharp decrease. The maximum activity was reached at a weaker concentration with long-chain acids. The depression of decarboxylation at concentrations exceeding the maximum is not due to aldehyde formation, but to penetration of the keto acid at a rate exceeding the decarboxylation velocity, and its consequent accumulation in the yeast cell. The reverse behaviour of straight-chain α-keto acids with yeast preparations and with intact yeast can only be explained by assuming that the cell membrane of intact yeast cells offers a barrier particularly to the penetration of short-chain keto acids. This assumption has been confirmed experimentally by potentiometric measurement of the penetration speeds. The only branched α-keto acid investigated, α-ketoisovaleric acid, was decarboxylated slower by yeast preparations than were the next straight-chain keto acids. With intact yeast, α-ketoisovaleric acid was decarboxylated less rapidly than α-ketobutyric acid.