In human plasma, the main agent of hydrolysis of the ester-type prodrug of levodopa, designated ONO-2160, is alpha-1-acid glycoprotein (AGP), which is a mixture of the F1*S and A variants at molar ratios of 3:1 to 2:1. In this study, the mechanism of AGP esterase-like activity was investigated by evaluating the contribution of the F1*S and A variants to ONO-2160 hydrolysis and identifying the AGP hydrolase active site. We found that although both variants hydrolyzed ONO-2160, their hydrolase activities were different. The intrinsic plasma clearance of the F1*S variant (0.441 mL/h/mg protein) was approximately 30 times higher than that of the A variant (0.0148 mL/h/mg protein), indicating that the F1*S variant contributed the most to AGP esterase-like activity. To identify the hydrolase active site of AGP, we performed inhibition studies of ONO-2160 hydrolysis using 12 AGP-binding drugs with various ligand-binding constants and binding selectivities to the two AGP variants. Inhibition of activity was positively correlated with the constant of ligand binding to the F1*S variant. In addition, compounds with high affinity to the F1*S variant inhibited ONO-2160 hydrolysis the most. Together, our data indicate that ONO-2160 is predominantly hydrolyzed by the F1*S variant at its ligand-binding site.
Keywords: alpha-1-acid glycoprotein (AGP); drug-binding protein; ester-type prodrug; genetic variant; plasma protein.