Tissue distribution of selective warfarin binding sites in the rat

Liver microsomes contain a specific warfarin binding site that is related to the target enzyme vitamin KO reductase [Thijssen HHW and Baars LGM, Biochem Pharmacol 38: 1115-1120, 1989]. In this study the distribution of the warfarin binder in the rat was investigated. Rats were given tracer doses of [14C]warfarin and tissue distribution was estimated after a time period. The selectivity of the distribution was verified by the ability of unlabeled warfarin to displace in vivo the tissue accumulated [14C]warfarin. The relation to the target enzyme vitamin KO reductase was verified by comparing the results with distribution behavior in the Scottish warfarin-resistant rat strain. The results show that in addition to liver various non-hepatic tissues accumulate warfarin. Among the tissues having a high accumulation ratio and a high rate of exchange by unlabeled warfarin are liver, pancreas, kidney, and salivary gland. Also arteria (aorta), bone, lung and spleen show exchangable [14C]warfarin accumulation. In HS rats the [14C]warfarin distribution was affected similarly for all tissues; lower levels of accumulation and higher rates of exchange by unlabeled warfarin. The tissue-bound warfarin was recovered predominantly in the microsomal fraction. Its release could only be accomplished in the presence of dithiothreitol and appeared to be stereoselective. The in vivo distribution pattern correlated with the number of warfarin binding sites in the tissue microsomes. The microsomal vitamin KO reductase activity did not always correlate to the binding capacity. The distribution was not affected by vitamin K deficiency. Warfarin-treated rats showed vitamin K epoxide accumulation in most of the organs having the warfarin binder.