The effects of vitamin K (phylloquinone: K1 and menaquinone-4: MK-4) on vascular calcification and their utilization in the arterial vessel wall were compared in the warfarin-treated rat model for arterial calcification. Warfarin-treated rats were fed diets containing K1, MK-4, or both. Both K1 and MK-4 are cofactors for the endoplasmic reticulum enzyme gamma-glutamyl carboxylase but have a structurally different aliphatic side chain. Despite their similar in vitro cofactor activity we show that MK-4 and not K1 inhibits warfarin-induced arterial calcification. The total hepatic K1 accumulation was threefold higher than that of MK-4, whereas aortic MK-4 was three times that of K1. The utilization of K1 and MK-4 in various tissues was estimated by calculating the ratios between accumulated quinone and epoxide species. K1 and MK-4 were both equally utilized in the liver, but the aorta showed a more efficient utilization of MK-4. Therefore, the observed differences between K1 and MK-4 with respect to inhibition of arterial calcification may be explained by both differences in their tissue bioavailability and cofactor utilization in the reductase/carboxylase reaction. An alternative explanation may come from an as yet hypothetical function of the geranylgeranyl side chain of MK-4, which is a structural analogue of geranylgeranyl pyrophosphate and could interfere with a critical step in the mevalonate pathway. Copyright 2003 S. Karger AG, Basel