Vitamins K: fat-soluble vitamins

Fourth and last of the commonly recognized fat-soluble vitamins are the vitamins K, so named because they are important for blood coagulation (coagulation is spelled with a K in German and in the Scandinavian languages). The K vitamins are close to the E vitamins chemically. Tocopherols are very similar to the naphthoquinones chemically; the K vitamins are actually naphthoquinones.

The basic structure of a naphthoquinone is shown below:

This is a 1,4 naphthoquinone. The most active form of vitamin K, a form which can be prepared synthetically, is 2-methyl-1,4-naphthoquinone. This is sometimes called menadione. It does not occur naturally nearly as abundantly as vitamin K1, which resembles menadione except that instead of having a methyl group in the 3 position, it has a phytol group. (Phytol is a 20 carbon mono-unsaturated primary alcohol which can be obtained from the saponification of chlorophyll.) Vitamin K 2 also resembles menadione except that in the 3 position it has a difarnesyl group. (Farnesol is similar to phytol; it is a 15 carbon primary alcohol with 3 unsaturated carbon atoms.) Several other naturally occurring vitamin K compounds have also been described. They are all either yellow oils or yellow crystalline solids, practically insoluble in water.

Vitamin K is manufactured in one form or another by bacteria, which do not ordinarily require it. The bacteria in the intestines of mammals produce enough so that normally the mammals have no need of it from outside sources. However, when absorption of fatty compounds is interfered with, as in intestinal disease or in liver disease involving interruption of bile flow, the vitamin becomes essential for man and mammals. Many birds require the vitamin, for their bacteria can not supply them with sufficient quantity.

Vitamin K is necessary for the clotting of the blood of higher animals. The reason for this is not clearly understood. To some extent opinions differ, but most authorities believe that this action of vitamin K is an indirect effect caused by the entrance into the blood of greater amounts of a substance prothrombin, the precursor of thrombin. Thrombin, which is regarded as an enzyme, is of primary importance in blood clotting.

Certainly the function of vitamin K can not be restricted to its effect on the clotting of blood. Obviously, the bacteria and the plants which produce it must have some need for it themselves. Experiments with isolated cells show that vitamin K is a very powerful protoplasmic clotting agent. This is not surprising in view of the fact that the clotting of protoplasm is in so many respects similar to the clotting of blood. Although menadione is but sparingly soluble in water, solutions of it cause a coagulation of the protoplasm of marine eggs. Menadione also produces clotting in muscle protoplasm.