Degradation of 4'-methylumbelliferyl 4-guanidinobenzoate catalyzed by human serum albumin.

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Abstract
The degradation of 4''-methylumbelliferyl 4-guanidinobenzoate (MUGB) was investigated in the presence of human serum albumin (HSA). HSA enhanced the rate of hydrolysis by a factor of 46.6 at pH 7.4. The pH-profile for the degradation of MUGB .cntdot. HSA complex showed an inflection point at about pH 9.0, suggesting the involvement of a group with a pKa of about 9. Inactivation of HSA with diethylpyrocarbonate (a histidine-specific reagent), and concomitant reactivation by hydroxylamine, indicated that histidine residues are involved in the esterase-like activity. Statistical analysis of the residual activity remaining in the partially modified HSA showed that only one histidine residue was critical for the activity among the 16 modifiable residues. In comparison with the HSA-catalyzed degradation of gabexate mesilate (GM) containing a guanidino group, the effects of site-specific ligands (warfarin, phenylbutazone and clofibric acid) on the HSA activity for MUGB degradation were examined. The results suggested that the active site for MUGB degradation is located far from the U-site and is different from the active site for GM degradation.

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