Studies on lipoamidase: characterization of the enzyme in human serum and breast milk.

Lipoamidase activity was detected in human serum with both lipoyllysine (epsilon-N-(DL-lipoyl)-L-Lysine) and lipoylPABA (N-DL-lipoyl-p-aminobenzoate) as substrates, whereas lipoamidase in human milk used lipoylPABA, but not lipoyllysine as substrate. This suggested that lipoamidase activities in serum and milk are due to different enzymes. Studies with activators and inhibitors suggested that lipoamidase in serum using lipoylPABA as substrate may be a different enzyme from that using lipoyllysine as substrate. We suggest that these lipoamidases are named lipoyllysine hydrolase (LLH) and lipoylPABA hydrolase (LPH), respectively. Serum LLH was activated by thiol compounds and EDTA and strongly inhibited by sulfhydryl reagents whereas serum LPH was inhibited by sulfhydryl reagents but not activated by thiol compounds or EDTA. Milk LPH was unaffected by these reagents. We suggest that serum LLH and possibly serum LPH are cysteine proteases. LLH was adsorbed on Concanavalin A-Sepharose, indicating that LLH was a glycoprotein.