Apoptosis-inducing activity of vitamin C and vitamin K.

Apoptosis-inducing activity of vitamins C and K and of their analogs are reviewed. Vitamin C shows both reducing and oxidizing activities, depending on the environment in which this vitamin is present. Higher concentrations of vitamin C induce apoptotic cell death in various tumor cell lines including oral squamous cell carcinoma and salivary gland tumor cell lines, possibly via its prooxidant action. The apoptosis-inducing activity of ascorbate is stimulated by Cu2+, lignin and ion chelator, and inhibited by catalase, Fe3+, Co2+ and saliva. On the other hand, at lower concentrations, ascorbic acid displays an antioxidant property, preventing the spontaneous and stress or antitumor agent-induced apoptosis. Sodium 5,6-benzylidene-L-ascorbate, intravenous administration of which induces degeneration of human inoperable tumors and rat hepatocellular carcinoma in vivo, induces apoptotic or non-apoptotic cell death, depending on the types of target cells. On the other hand, elevation of intracellular concentration of ascorbic acid by treatment with ascorbate 2-phosphate or dehydroascorbic acid makes the cells resistant to the oxidative stress-induced apoptosis. Vitamin K2, which has a geranylgeranyl group as a side chain,and vitamin K3 induces apoptosis of various cultured cells including osteoclasts and osteoblasts, by elevating peroxide and superoxide radicals. Synergistic apoptosis-inducing actions have been found between vitamins C and K, and between these vitamins and antiproliferative agents. The possible therapeutic application of these vitamins is discussed.     

Chromatographic separation of vitamin D3 sulfate and vitamin D3.

This paper describes a simple chromatographic technique on Sephadex LH20 for the separation of vitamin D3 sulfate from free vitamin D3 and its metabolites. This technique has been used in the study of vitamin D3 sulfate metabolism in rats. Seven hours after injection of vitamin D3 sulfate (35S or 35S and 3H) only the peak of vitamin D sulfoconjugate was found in chromatographic elution of serum extracts.     

Biosynthesis of vitamin B2.

GTP cyclohydrolase II catalyzes the hydrolytic release of formate and pyrophosphate from GTP producing 2,5-diamino-6-ribosylamino-4(3H)-pyrimidinone 5'-phosphate, the first committed intermediate in the biosynthesis of riboflavin. The enzyme was shown to contain one zinc ion per subunit. Replacement of cysteine residue 54, 65 or 67 with serine resulted in proteins devoid of bound zinc and unable to release formate from the imidazole ring of GTP or from the intermediate analog, 2-amino-5-formylamino-6-ribosylamino-4(3H)-pyrimidinone 5'-triphosphate. However, the mutant proteins retained the capacity to release pyrophosphate from GTP and from the formamide-type intermediate analog. The data suggest that the enzyme catalyzes an ordered reaction in which the hydrolytic release of pyrophosphate precedes the hydrolytic attack of the imidazole ring. Ring opening and formate release are both dependent on a zinc ion acting as a Lewis acid, which activates the two water molecules involved in the sequential hydrolysis of two carbon-nitrogen bonds.     

Regulation of vitamin D metabolism.

Vitamin D can be regarded as a prohormone and its most potent metabolite, 1, 25-dihydroxyvitamin D₃, a hormone which mobilizes calcium and phosphate from bone and intestine. In true hormonal fashion, the biosynthesis of 1, 25-dihydroxyvitamin D₃ by kidney mitochondria is feed-back regulated by serum calcium and serum phosphorus levels. The lack of calcium brings about a secretion of parathyroid hormone which stimulates 1, 25-dihydroxyvitamin D₃ synthesis while low blood phosphorus stimulates 1, 25-dihydroxyvitamin D₃ synthesis even in the absence of the parathyroid glands. For such regulation to occur, vitamin D must be present probably because 1, 25-dihydroxyvitamin D₃ itself is needed for the regulation. The molecular and cellular mechanisms whereby 1, 25-dihydroxyvitamin D₃ synthesis is regulated are unknown despite many recent reports. Likely the elucidation of these mechanisms must await a detailed investigation of the enzymology of the renal 25-hydroxyvitamin D₃-1α-hydroxylase. In addition to the regulation at the 25-hydroxyvitamin D₃-1α-hydroxylase step, vitamin D metabolism is regulated at the hepatic vitamin D-25-hydroxylase level. This regulation is a suppression of the hydroxylase by the hepatic level of 25-hydroxyvitamin D₃ itself by an unknown mechanism. Much remains to be learned concerning the regulation of this newly discovered endocrine system but already the findings are not only relevant to calcium homeostasis but also to an understanding of a variety of metabolic bone diseases.