Effect of vitamin B6 deficiency on collagen metabolism in rats

The Role of B-Vitamins in Bone Health and Disease in Older Adults

The risk of osteoporosis and bone fractures increases with age. Several other factors are also related to bone disease including gender, race/ethnicity, physical activity, alcohol, smoking, estrogen, and calcium and vitamin D. B-vitamins (folate, B12, and B6) are also emerging dietary factors related to bone health, both individually and through their action on influencing total plasma homocysteine concentrations (tHcy). The primary objective of this review is to summarize the available data on B-vitamins and bone health, highlighting clinical trials and observational data. In populations without folic acid fortification, the totality of evidence suggests that elevated tHcy has a small but significant association with bone fracture risk and bone quality but not on bone mineral density (BMD) or bone turnover biomarkers. Very little supportive evidence exists for a direct role of folate for either BMD or fracture risk; however, the data available are quite limited. Meta-analyses and some cross-sectional and cohort studies suggest a small but significant role of vitamin B12 status on risk of fracture but not on BMD. The mechanism by which tHcy and B12 may influence bone health is not well characterized but may be through modulation of collagen cross-linking or through altering osteoclasts or osteoblasts. Much more data are needed-particularly the role that each vitamin directly has on bone, or whether the vitamins only exert their effect though tHcy concentrations. Nevertheless, consistent findings across different populations with different study designs suggest a role for tHcy and B12 in reducing fracture risk.

Changes of glucose metabolism and skin-collagen neogenesis in vitamin B6 deficiency

The mechanism of pellagrous changes in skin caused by a deficiency of vitamin B6 was studied in respect to neogenesis of proline in skin collagen and glucose metabolism. In vitamin B6 deficiency the insulin/glucagon coefficient in serum decreased significantly from 3.02 to 2.32, indicating a metabolic change towards gluconeogenesis. A deficiency of vitamin B6 caused a decrease in the levels of vitamin B6-dependent enzymes, such as ornithine aminotransferase, alanine aminotransferase, and aspartate aminotransferase, which also contribute to gluconeogenesis. Because the conversion of ornithine to proline via pyrroline-5-carboxylate was suppressed due to the decrease in ornithine aminotransferase activity, the amount of proline in the skin collagen fraction also decreased significantly in vitamin B6-deficient rats compared with the pair-fed control. These results suggest that the pellagrous lesions in vitamin B6-deficiency are caused by an impaired synthesis of proline from ornithine, which results in the suppression of collagen neogenesis in the skin.

Reactivity of a functional carbonyl moiety in bovine aortic lysyl oxidase. Evidence against pyridoxal 5'-phosphate

Previous studies have pointed towards a cofactor role for pyridoxal 5'-phosphate (PLP) in lysyl oxidase, the enzyme that generates the peptidyl aldehyde precursor to the lysine-derived cross-linkages in elastin and collagen. The nature of a carbonyl moiety in purified bovine aortic lysyl oxidase was explored in the present study. A PLP dinitrophenylhydrazone could not be isolated from lysyl oxidase, although corresponding preparations of aspartate aminotransferase, a PLP-dependent enzyme, yielded this derivative, as revealed by h.p.l.c. Analysis of lysyl oxidase for PLP after reduction of the enzyme by NaBH4, a procedure that converts PLP-protein aldimines into stable 5'-phosphopyridoxyl functions, also proved negative in tests using monoclonal antibody specific for this epitope. Lysyl oxidase was competitively inhibited by phenylhydrazine, and inhibition became irreversible with time at 37 degrees C, displaying a first-order inactivation rate constant of 0.4 min-1 and KI of 1 microM. [14C]Phenylhydrazine was covalently incorporated into the enzyme in a manner that was prevented by prior modification of the enzyme with beta-aminopropionitrile, a specific active-site inhibitor, and which correlated with functional active-site content. The chemical stability of the enzyme-bound phenylhydrazine exceeded that expected of linkages between PLP and proteins. The absorption spectrum of the phenylhydrazine derivative of lysyl oxidase was clearly distinct from that of the phenylhydrazone of PLP. It is concluded that lysyl oxidase contains a carbonyl cofactor that is not identical with PLP and that is bound to the enzyme by a stable chemical bond.

Impaired collagen maturity in vitamins B2 and B6 deficiency--probable molecular basis of skin lesions

Solubility of collagen was increased and the proportion of insoluble collagen was reduced in the skin of both riboflavin as well as pyridoxine-deficient rats. Collagen content of the skin, and aldehyde concentration of salt-soluble collagen were also lower in the deficient groups. The alpha:beta subunit ratio of salt-soluble collagen was higher in riboflavin deficiency. In food-restricted weight-matched control groups, similar changes in collagen solubility, but of lesser magnitude were observed. Both food restriction and riboflavin deficiency decreased plasma PLP concentration. Increase in the solubility of collagen, decrease in the aldehyde content of soluble collagen and increase in the alpha:beta subunit ratio of soluble collagen, suggest that the maturation of collagen may be affected in pyridoxine or riboflavin deficiency. These molecular events may be etiologically related to the pathogenesis of the skin lesions in vitamin B2 or B6 deficiency.