Human serum lipoamidase.

Thirty-two human serum specimens were assayed for lipoamidase (lipoyl-4-aminobenzoate hydrolase) activity. All sera had lipoamidase activities. This substrate was newly synthesized by us and had a satisfying purity as evaluated by HPLC-fluorimetric detection. Product (p-aminobenzoate) liberated was determined directly by the HPLC-fluorimetric method. Liberation of the product was linearly continued for 6 h. The pH optimum of serum lipoamidase was found to be 7.0. The effect of substrate concentration on human serum lipoamidase activity was examined and the reaction was saturated at 0.1 mmol/l. The sera obtained were from individuals aged from 1 to 8 years. The mean value of serum lipoamidase activity was found to be 1.50 U/l (SD 1.037, range 0.04-3.75, n = 32). The difference of sex effects was analyzed and no significant difference was found (males: n = 14, mean 1.48, SD 1.162, range 0.04-3.75; females: n = 18, mean 1.52, SD 0.963, range 0.48-3.51) among this age group. Biotinidase activity was also determined in these 32 serum specimens and the correlation was examined. The mean biotinidase activity was 3.16 U/l (SD 2.567, range 0.35-9.37). The correlation coefficient (r) between lipoamidase activity and biotinidase activity was 0.8931. Although the physiological significance of lipoamidase has not been known, the enzyme might play an important role in recycling of lipoate as biotinidase does.