Intestinal absorption of different vitamin K compounds in the horse

Intestinal absorption of different vitamin K compounds in the horse

Vitamin K is involved in many physiological processes beyond that of blood coagulation, including bone metabolism, energy metabolism, spermatogenesis, apoptosis and immunity. There are several forms of vitamin K phylloquinone (K1), synthesized by green plants; menaquinones (MKs also known as K2) synthesized by bacteria; menadione (K3) which is the synthetic form of the vitamin and is routinely added to animal diets] but there is no consensus of their efficacy in vivo. As absorption is the first step in vitamin K metabolism, the objective of this study was to determine the absorption efficiency of different isoforms of vitamin K in the horse. Twelve mature geldings were paired and allocated to 6 groups in a random crossover design. Sampling was undertaken over 6 experimental periods. There were 6 treatments consisting of a control, K1, K2 (in the form of MK-4), K3 and KQ (QAQ; Quinaquanone a soluble form of K1 and K2 (10:1). These were administered as a 200 mg oral bolus to each horse. Blood sampling was undertaken for 480 min. In the last treatment K1 (200 mg) was administered intravenously and blood samples collected at designated intervals for 2 h. Plasma samples were analyzed by HPLC for K1, MK-4 and K3 concentrations. Plasma K1 concentrations differed significantly across time and between treatments (P < 0.001) with the highest plasma K1 concentrations occurring with the KQ treatment compared with the other treatments (P < 0.0001), with K1 peaking second highest. Both KQ and K1 showed no detectable conversion to K3 or MK4 in plasma. Moreover, the administration of K3 showed that plasma K3 was well absorbed, but there was no detectable conversion to MK4 in plasma. Pharmacokinetic parameters [area under the curve (AUC), time to maximum (tmax) and maximum concentration (Cmax)] were derived for each treatment and bioavailability calculated for oral treatments relative to K1 administered intravenously (100%). The bioavailability was 0.45% for QAQ and 0.14% for K1. The results of this study demonstrate that the horse appears to almost exclusively have K1 in plasma in contrast other species. This suggests that there is no specific conversion of K1 to K3 or K3 to MK-4 in the horse, contrary to what occurs in some other mammals. The soluble form of the vitamin, KQ was the most efficiently absorbed and should be evaluated in further studies that examine metabolism of vitamin K in the horse, especially bone metabolism.

Abstract #11.