Department of Pharmaceutical Sciences, College of Pharmacy, University of Michigan, 428 Church Street, Ann Arbor, Michigan 48109-1065, United States.
Mol Pharm. 2010 Dec 6;7(6):2362-8. doi: 10.1021/mp100300k. Epub 2010 Nov 11.
Human valacyclovirase (hVACVase) is a prodrug-activating enzyme for amino acid prodrugs including the antiviral drugs valacyclovir and valganciclovir. In hVACVase-catalyzed reactions, the leaving group of the substrate corresponds to the drug moiety of the prodrug, making the leaving group effect essential for the rational design of new prodrugs targeting hVACVase activation. In this study, a series of valine esters, phenylalanine esters, and a valine amide were characterized for the effect of the leaving group on the efficiency of hVACVase-mediated prodrug activation. Except for phenylalanine methyl and ethyl esters, all of the ester substrates exhibited a relatively high specificity constant (k(cat)/K(m)), ranging from 850 to 9490 mM(-1)·s(-1). The valine amide Val-3-APG exhibited significantly higher K(m) and lower k(cat) values compared to the corresponding ester Val-3-HPG, indicating poor specificity for hVACVase. In conclusion, the substrate leaving group has been shown to affect both binding and specific activity of hVACVase-catalyzed activation. It is proposed that hVACVase is an ideal target for α-amino acid ester prodrugs with relatively labile leaving groups while it is relatively inactivate toward amide prodrugs.
人丙氧鸟苷酶（hVACVase）是一种前体药物激活酶，可激活包括抗病毒药物伐昔洛韦和缬更昔洛韦在内的氨基酸前体药物。在 hVACVase 催化的反应中，底物的离去基团对应于前体药物的药物部分，因此对于针对 hVACVase 激活的新前体药物的合理设计，离去基团效应至关重要。在这项研究中，我们对一系列缬氨酸酯、苯丙氨酸酯和缬氨酸酰胺进行了表征，以研究离去基团对 hVACVase 介导的前体药物激活效率的影响。除了苯丙氨酸甲酯和乙酯外，所有的酯类底物都表现出相对较高的特异性常数（k(cat)/K(m)），范围为 850 到 9490mM(-1)·s(-1)。与相应的酯 Val-3-HPG 相比，缬氨酸酰胺 Val-3-APG 表现出明显更高的 K(m)和更低的 k(cat)值，表明其对 hVACVase 的特异性较差。总之，研究表明，底物的离去基团既影响 hVACVase 催化的激活的结合，又影响其特异性活性。可以提出 hVACVase 是相对不稳定离去基团的α-氨基酸酯前体药物的理想靶标，而对于酰胺前体药物，则相对不活跃。
