Michaelis–Menten Kinetics of Renal Tubular Secretion of D-(−)-p-Methyl Mandelic Acid and D-(−)-p-Ethyl Mandelic Acid in Rats

Abstract

Studies were carried out to determine the Michaelis–Menten kinetic parameters (Vm and Vm) of renal tubular secretion of para-alkylated mandelic acids, namely, D-(−)-p-methyl mandelic acid and D-(−)-p-ethyl mandelic acid, in rats. It is shown that the maximum initial secretion rates (Vm) of these compounds are similar, and they share a common "carrier" mechanism for their renal tubular secretion. Since the value of the intravenous dose (μmole/kg.) required to produce half the maximum secretion rate (Vm) for D-(−)-p-ethyl mandelic acid is found to be about one-half that for D-(−)-p-methyl mandelic acid, it is concluded that the affinity of the former for the "carrier" is approximately twice that of the latter. Evidence was obtained to indicate that the para-alkylated mandelic acids used in this study are secreted by the same mechanism responsible for the secretion of D-(−)-mandelic acid and its homologs such as tropic acid. Although the volume of distribution available in rats for these para-alkylated mandelic acids is demonstrated to be similar, it is shown to be strikingly smaller than that for D-(−)-mandelic acid and its homologs obtained by introducing methylene group(s) in the side chain of the mandelic acid molecule. The biological significance of the data obtained for the "model compounds" is discussed.