Competitive relationship between protocatechuic acid and p-aminosalicylic acid for a cellular transport mechanism.

Hubbard, Jerry S. (Oklahoma State University, Stillwater), and Norman N. Durham. Competitive relationship between protocatechuic acid and p-aminosalicylic acid for a cellular transport mechanism. J. Bacteriol. 82:361-369. 1961.-The oxidation of protocatechuic acid by a Flavobacterium is inhibited by p-aminosalicyclic acid regardless of whether the organism is grown on protocatechuic acid or sequentially induced to protocatechuic acid by growth on p-aminobenzoic acid. Depletion of the substrate from the medium by the cell suspension is dependent, within defined limits, on the inhibitor to substrate ratio, and the inhibition can be overcome by addition of excess substrate. However, this competitive effect is not observed in high inhibitor to substrate ratios. p-Aminosalicylic acid did not affect the rate or extent of oxidation, carbon dioxide evolution, or formation of beta-ketoadipic acid during degradation of protocatechuic acid by cell extracts. The results suggest that p-aminosalicylic acid antagonizes the oxidation of protocatechuic acid by the cell suspension by competing with the substrate for a specific transport mechanism, thereby regulating the entry and internal accumulation of the substrate. The lack of a competitive effect in high inhibitor to substrate ratios could be interpreted as an indication that the mechanism for accumulating the substrate may consist of more than one active transport system.