Conjugation of p-Aminohippurate by the Kidney and Effective Renal Plasma-flow

Abstract

THE clearance of p-aminohippurate has become the standard method for the determination of effective renal plasma-flow, with the assumption that there is no conjugation of p-aminohippurate during its passage through the kidney from the blood to the urine. The metabolism of this substance in the body is considered to be similar to that of sulphanilamide which it resembles, namely, acetylation of the para amino group. p-Acetylaminohippurate and p-aminohippurate have similar clearances in the dog and man, so the former may be expected to depress maximal tubular excretion of the latter, but as neither p-acetylaminohippurate nor any metabolite in which the para amino group is substituted is estimated by the technique used for p-aminohippurate, conjugation at sites other than in the kidney should not affect the clearance of the latter. With sulphanilamide, conjugation occurred only in the liver of rabbits and rats, in the liver and spleen in cats, and not at all in the dog (cf. ref. 1). p-Aminohippurate itself was not conjugated by rabbit² or dog³ kidney slices in vitro, although some conjugation was found with rat⁴ and guinea pig² kidney slices. From the evidence available, Smith¹ came to the conclusion that conjugation in the kidney did not occur to any significant extent in the species studied (rat, rabbit, cat, dog and man), although it has been suggested by others that conjugation could explain low clearances of p-aminohippurate with falling plasma concentrations in man⁵.