The influence of vitamin K1 on the structure and phase behaviour of model membrane systems.

Vitamin K1 is a component of the Photosystem I of plants which constitutes the major dietary form of vitamin K. The major function of this vitamin is to be cofactor of the microsomal gamma-glutamylcarboxylase. Recently, novel roles for this vitamin in the membrane have been postulated. To get insight into the influence of vitamin K1 on the phospholipid component of the membrane, we have studied the interaction between vitamin K1 and model membranes composed of dimyristoylphosphatidylcholine (DMPC) and dielaidoylphosphatidylethanolamine (DEPE). We utilized high-sensitivity differential scanning calorimetry and small-angle X-ray diffraction techniques. Vitamin K1 affected the thermotropic properties of the phospholipids, broadened and shifted the transitions to lower temperatures, and produced the appearance of several peaks in the thermograms. The presence of the vitamin gave rise to the formation of vitamin-rich domains which were immiscible with the bulk phospholipid in both the gel and the liquid-crystalline phases. Vitamin K1 was unable to alter the lamellar organization of DMPC, but we found that it produced an increase in the interlamellar repeat spacing of DMPC at 10 degrees C. Interestingly, vitamin K1 promoted the formation of inverted hexagonal HII structures in the DEPE system. We discuss the possible implications that these vitamin K1-phospholipid interactions might have with respect to the biological function of the vitamin.