Identification of 8 alpha,25-dihydroxy-9,10-seco-4,6,10(19)-cholestatrien-3-one as a product of metabolism of 25-hydroxycholecalciferol by liver microsomes

Exogenous 25-hydroxycholecalciferol does not attenuate salt-induced hypertension

We have reported that an inverse relationship exists between blood pressure and plasma concentration of 25-hydroxyvitamin D, the precursor of the hormonal form of vitamin D, for Dahl salt-sensitive rats fed a high salt diet. Plasma 25-hydroxyvitamin D concentrations decreased with time on the diet, as blood pressure increased. Experiments were conducted to determine whether the blood pressure increase of salt-sensitive rats fed a high salt diet could be attenuated by exogenous 25-hydroxycholecalciferol. Dahl salt-sensitive rats were fed a high salt diet and administered exogenous 25-hydroxycholecalciferol via subcutaneously implanted Alzet pumps. Exogenous 25-hydroxycholecalciferol (various doses from 28 to 80 microg/kg body weight-day) had no significant effect on the blood pressure of vitamin D-replete rats fed a high salt diet for 15 days. When exogenous 25-hydroxycholecalciferol (28 and 60 microg/day-kg body weight) was administered to vitamin D-depleted salt-sensitive rats, plasma 25-hydroxyvitamin D concentrations of the rats fed a low salt diet (26 +/- 2 and 59 +/- 6 nM) were proportional to the 25-hydroxycholecalciferol concentration in the pumps. Plasma 25-hydroxyvitamin D concentrations of the rats fed a high salt diet (18 +/- 1 and 23 +/- 3 nM) were not proportional to the 25-hydroxycholecalciferol concentration in the pumps, but were inversely proportional to the blood pressure of the rats. These data indicate no ameliorating effect of exogenous 25-hydroxycholecalciferol on salt-induced hypertension, but accelerated metabolism and/or clearance of 25-hydroxycholecalciferol in salt-induced hypertension.

Synthesis and function of 8 alpha,25-dihydroxy-3-oxoneocholecalciferol in liver

25-Hydroxycholecalciferol (25-OHD3) is converted to 8 alpha,25-dihydroxy-3-oxoneocholecalciferol [8,25-(OH)2-3-oxoneo-D3] by liver microsomes, alveolar macrophages and myeloid leukemia cells. The characteristics of this reaction in liver microsomes have been determined. Omission of an NADPH-generating system or NADH resulted in a greater than 75% reduction in the production of 8,25-(OH)2-3-oxoneo-D3. In the absence of the cytosolic fraction, 25-OHD3 was converted to products that comigrated with 8,25-(OH)2-3-oxoneo-D3 on a silica column developed with hexane-isopropanol, thereby preventing quantitation. Production of 8,25-(OH)2-3-oxoneo-D3 was unaffected by EDTA and was stimulated by N,N'-diphenyl-p-phenylenediamine. Both progesterone and pregnenolone inhibited production of 8,25-(OH)2-3-oxoneo-D3; inhibition by progesterone was greater than that by pregnenolone. 8,25-(OH)2-3-Oxoneo-D3 did not bind the thymus receptor for 1,25-dihydroxycholecalciferol [1,25-(OH)2D3] at concentrations 10-fold higher than that of 1,25-(OH)2D3. The lack of affinity of 8,25-(OH)2-3-oxoneo-D3 for the 1,25-(OH)2D3 receptor suggests that this metabolite is a degradative product of 25-OHD3, which might be produced when 25-OHD3 concentrations in the liver are excessive. Synthesis of this metabolite in the liver may be catalyzed by enzymes that also metabolize other steroids.