Lipoamidase and biotinidase activities in the rat: tissue distribution and intracellular localization

Lipoamidase (not yet given an EC number) activity was measured in various rat tissues using two different substrates, one natural, lipoyllysine (epsilon-N-(D,L-lipoyl)L-lysine) and one artificial, lipoyl-p-aminobenzoic acid (N-D,L-lipoyl-p-aminobenzoic acid). Biotinidase, EC 3.5.1.12, was measured in the same tissue with the artificial substrate, biotinyl-p-aminobenzoic acid (N-D-biotinyl-p-aminobenzoic acid). Lipoamidase measured as lipoyl-p-aminobenzoic acid hydrolase activity had two pH optima, at pH 6.0 and pH 9.5, in liver homogenate, but only one pH optimum at pH 6.0 in rat plasma. Lipoamidase measured as lipoyllysine hydrolase activity had a pH optimum at pH 5.5 both in liver homogenate and plasma. Similarly, biotinidase shows a single pH optimum at pH 6.0 in liver homogenate and plasma. The properties of lipoyllysine hydrolase and biotinidase were similar with respect to thermostability, pH stability and inhibition pattern, and their properties differed from those of lipoyl-p-aminobenzoic acid hydrolase. Lipoyllysine hydrolase and biotinidase activities were highest in kidney, liver and blood plasma, whereas lipoyl-p-aminobenzoic acid hydrolase activities were highest in liver, brain and kidney. Lipoyllysine hydrolase and biotinidase activities were found mainly in the liver microsomal fraction, and lipoyl-p-aminobenzoic acid hydrolase was recovered from the microsomal fraction and to a small extent from the mitochondrial fraction. These results indicate that liver lipoyl-p-aminobenzoic acid hydrolase is an enzyme protein which differs from lipoyllysine hydrolase, and the data also indicate that liver lipoyllysine hydrolase and biotinidase are the same enzyme protein.