A survey of rat tissues for phylloquinone epoxidase activity

Vitamin K status in human tissues: tissue-specific accumulation of phylloquinone and menaquinone-4

We measured the vitamin K status in postmortem human tissues (brain, heart, kidney, liver, lung, pancreas) to see if there is a tissue-specific distribution pattern. Phylloquinone (K1) was recovered in all tissues with relatively high levels in liver, heart and pancreas (medians, 10.6 (4.8), 9.3 (4.2), 28.4 (12.8) pmol(ng)/g wet weight tissue); low levels (< 2 pmol/g) were found in brain, kidney and lung. Menaquinone-4 (MK-4) was recovered from most of the tissues; its levels exceeded the K1 levels in brain and kidney (median, 2.8 ng/g) and equalled K1 in pancreas. Liver, heart and lung were low in MK-4. The higher menaquinones, MK-6-11, were recovered in the liver samples (n 6), traces of MK-6-9 were found in some of the heart and pancreas samples. The results show that in man there are tissue-specific, vitamin-K distribution patterns comparable to those in the rat. Furthermore, the accumulation of vitamin K in heart, brain and pancreas suggests a hitherto unrecognized physiological function of this vitamin.

Tissue distribution and warfarin sensitivity of vitamin K epoxide reductase

The distribution of vitamin K epoxide reductase activity and its sensitivity to warfarin have been examined in whole microsomes from tissues of both control and warfarin-resistant strain rats. The distribution of activity roughly paralleled that previously shown for the vitamin K-dependent carboxylase. Activity on a per gram tissue basis was highest in kidney, adrenal, spleen, lung, testes, and epididymis at a level about 1/20th of that present in liver microsomes. Vitamin K quinone formation by microsomes from warfarin-resistant rats was approximately half that of control strain samples. In addition, hydroxy vitamin K was formed by warfarin-resistant strain microsomes to about the same extent as vitamin K quinone in all tissues. The Km values for dithiothreitol (DTT) and vitamin K epoxide were similar in all tissues (range = 0.1-0.2 mM DTT at 40 microM vitamin K epoxide, and 10-30 microM vitamin K epoxide at 2 mM DTT). The sensitivities to warfarin were similar for all control strain rat tissues (I50 = 10-20 microM at 2 mM DTT and 40 microM vitamin K epoxide) and similarly elevated for all warfarin-resistant rat tissues (I50 = 30 to greater than 80 microM). These results suggest that the identical enzyme is expressed in all tissues and that tissue specific isozymes do not occur.

Post-translational carboxylation of preprothrombin

In summary, in this review on the function of vitamin K in post-translational modification of precursor proteins by carboxylation of certain glutamyl residues, I have tried to cover in particular the recent work on the reaction, the enzymes involved and the mechanisms being considered. In doing this I have also considered vitamin K, its discovery, its functional form and the possible relation of its metabolism to the carboxylation reaction. Equally the various vitamin K-dependent gla-containing proteins currently known have been described. The carboxylation of synthetic small molecule exogenous substrates and the synthesis and metabolism of the products of carboxylation are of great help in studying the reaction. Structural specificity of vitamin K analogs in vivo and in vitro has been compared and the use of various antagonists in vivo and in vitro considered in attempts to gain an understanding of the overall reaction. The reactions subsequent to carboxylation, e.g., the activation of prothrombin to thrombin via serine proteases and the related activation of the other vitamin K-dependent proteins have not been considered in this review. The review has not covered prothrombin or other vitamin K-dependent protein isolation, nor the determination of these proteins. As the vitamin K-dependent protein carboxylation story has developed over the past six years, a number of reviews have been written which help in keeping up with the various aspects of the field as it has expanded. These reviews refer to many of the papers I have had to eliminate due to space limitations. They are referenced as 469-489. The review is in no sense comprehensive and many papers have been missed or only mentioned. I have tried to concentrate on the more recent work and, thus, much of the very fine work of the 1940's on vitamin K chemistry is hardly mentioned. Some redundancy has been built into the organization of the review so that a reader can obtain a reasonable view of any one section without having to search the whole review for all possible relevant information on any particular part of the field.

Mechanism of action of vitamin K: synthesis of gamma-carboxyglutamic acid

Vitamin K (2-methyl-3-phytyl-1,4-naphthoquinone) is required for the synthesis of prothrombin, Factor VII, Factor IX, Factor X, and a number of newly discovered proteins. These plasma proteins participate in calcium-dependent phospholipid membrane interactions which are mediated through the presence of gamma-carboxyglutamyl residues in their amino-terminal region. Vitamin K is required for the postribosomal conversion of glutamyl residues in liver precursors of these proteins to gamma-carboxyglutamyl residues in the completed plasma proteins. In the absence of vitamin K, or in the presence of vitamin K antagonists, animals produce plasma forms which lack the carboxylated residue. These proteins are nonfunctional because of their lack of phospholipid interaction. The vitamin K-dependent carboxylase which carries out this reaction has been studied in rat liver microsomal preparations where it will carboxylate the endogenous precursor proteins. Low-molecular-weight glutamyl-containing peptide substrates, such as Phe-Leu-Glu-Glu-Leu, which are homologous to regions of the prothrombin precursor, will also serve as substrates for the detergent-solubilized enzyme. This enzyme has been shown to require the reduced form of the vitamin and O2 but no ATP or a biotin-containing protein for its activity. The same microsomal preparations will also convert vitamin K to its 2,3-epoxide, and it is possible that activity may be related to the role of the vitamin in driving the carboxylase reaction.