25-Hydroxylation of vitamin D2 and D3 was studied in subcellular fractions from human liver, using a technique based on isotope dilution-mass spectrometry. The mitochondrial fraction fortified with isocitrate catalysed 25-hydroxylation of vitamin D3 at a rate of about 10 pmol/mg protein X min. Under the same conditions, the rate of 25-hydroxylation of vitamin D2 was less than 2 pmol/mg protein X min. Crude microsomes fortified with NADPH catalysed 25-hydroxylation of vitamin D3 to a very low extent, and this activity was not linear with the amount of microsomal protein. A higher rate of conversion was obtained with a partially purified cytochrome P-450 fraction in the presence of NADPH-cytochrome P-450 reductase and NADPH. This fraction also catalysed 25-hydroxylation of 1 alpha-hydroxyvitamin D3 and 5 beta-cholestane-3 alpha, 7 alpha, 12 alpha-triol. 25-Hydroxylation of vitamin D2 could not be detected, neither with crude microsomes, nor with the microsomal cytochrome P-450 fraction. Since the assay for 25-hydroxyvitamin D2 was less sensitive than that for 25-hydroxyvitamin D3, these experiments do not rule out the presence of some 25-hydroxylase activity towards vitamin D2 in the microsomes. The results are discussed in relation to previous work in which a lower toxicity has been reported for vitamin D2 than for vitamin D3 in some mammalian species.

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The absorption of a pharmacological dose of vitamin D, from two different lipid vehicles, peanut oil, containing long chain fatty acids, and a medium chain triglyceride was compared. Serial measurements of the serum concentration of vitamin D, after dosage were made. The serum levels of 25-hydroxyvitamin D,, the major circulating vitamin D3 metabolite, were also determined. The analytical methods used were based on HPLC. In the fasting state, the serum levels of vitamin D, were significantly higher after administration in peanut oil than after administration in the medium chain triglyceride. When the vitamin D3 dose was ingested together with food no difference between the two formulations was observed. Only small inter-formulation differences in serum 25-hydroxyvitamin D, levels were detected. The results indicate that the presence of long chain fatty acids facilitates the absorption of vitamin D, KEY WORDS Vitamin D3 Lipid vehicle Absorption

Normocalcaemic male stone formers, 31-51 years old (n = 108) on a free diet, were divided into a hypercalciuric group (n = 47) with calcium excretion rates higher than 8.0 mmol/24 h, a normocalciuric group (n = 32) with calcium excretion rates below 6.1 mmol/24 h and an intermediate group (n = 29). There were no statistically significant differences between the hypercalciuric and the normocalciuric groups with respect to serum levels of calcium, phosphate, creatinine, urate, ALAT, albumin, PTH, 1,25-dihydroxyvitamin D or urinary excretion of cAMP. The group of patients with high calcium excretion had significantly higher serum levels of 25-hydroxyvitamin D3 (75 +/- 4 nmol/l) than the group with low calcium excretion (57 +/- 4 nmol/l) (p less than 0.002), while the group of patients with intermediate calcium excretion had 25-hydroxyvitamin D3 levels between the other two groups (69 +/- 4 nmol/l). A highly accurate method based on isotope dilution-mass spectrometry was used to assay 25-hydroxyvitamin D3. Of the patients with hypercalciuria (n = 47), seven were classified as hyperabsorbers on the basis of calcium load tests. These patients were found to have even higher serum levels of 25-hydroxyvitamin D3 (108 +/- 10 nmol/l)--significantly higher than that of the hypercalciuric patients as a whole. The above study was carried out in March 1983. In September, the group of patients with high urinary calcium excretion also had significantly higher levels of 25-hydroxyvitamin D3 than the group of patients with low calcium excretion.(ABSTRACT TRUNCATED AT 250 WORDS)