The biological synthesis of hippuric acid in vitro

Abstract

The mechanism of the synthesis of hippuric acid in viva is interesting from several points of view. There is the intrinsic interest in a compound found in the urine of many animals, an interest which is heightened by the use of the rate of hippuric acid excretion following the administration of benzoic acid as a clinical test of liver function. This synthesis is interesting also from the point of view of physiological energetics. The formation of hippuric acid from glycine and benzoic acid is attended by a gain in free energy (Table I). In other words the tendency of the reaction, if allowed to proceed spontaneously at 25° or 38°, is not toward synthesis but toward practically complete hydrolysis of hippuric acid (Table III). Yet when benzoic acid is fed, hippuric acid is rapidly synthesized. This synthesis also occurs and can be measured, as shown below, when liver slices are suspended in Ringer’s solution containing low concentrations of benzoic acid and glycine. More than half the benzoic acid is converted to hippuric acid. From the thermodynamic data it may be deduced that the enzymatic synthesis of hippuric acid cannot be simply the reverse of its hydrolysis. The hydrolysis can proceed spontaneously; the synthesis must be coupled with an energy-yielding reaction