fate of tetrahydrolipstatin (1) incubated with humancarboxyl-esterlipase (HCEL) was investigated. The primary metabolite was identified as δ-(N-formyl-L-leucyloxy)-β-hydroxy acid 2a with conserved configuration. It is formed by attack of the active-site serine of HCEL at the carbonyl C-atom of the β-lactone ring of 1 followed by hydrolysis of the intermediate serine ester. Further products isolated