Liberation of lipoate by human serum lipoamidase from bovine heart pyruvate dehydrogenase

Lipoamidase, which hydrolyses such substrates as lipoamide, lipoylmethyl ester, lipoyllysine, and lipoyl 4-aminobenzoate (LPAB), was purified from human serum through use of synthetic substrate LPAB. The purified human serum lipoamidase showed lipoyllysine hydrolase activity (Km=435 μM, Vmax=64.5 nmol/min per mg of protein). The purified enzyme did not liberate the free form of lipoic acid from bovine heart pyruvate dehydrogenase (PDH). PDH was hydrolyzed quantitatively by proteinase K to lipoyllysine, which was determined by the HPLC method. Although liberation of lipoate from various lengths of lipoyl-peptides has not been tested yet, it is likely that lipoamidase requires proteinase(s) before the liberation of free lipoic acid from the enzymes.