Identification of enzyme activities involved in vitamin D3 metabolism in Solanum malacoxylon

In animals vitamin D3 is made from 7-deydrocholesterol through the action of UV light in the skin; the activation process of vitamin D3 involves a 25-hydroxylation in the liver, followed by a 1a-hydroxylation in the kidney, leading to 1a,25(OH)2D3, the hormone active on the regulation of calcium homeostasis. Vitamin D3 and the related hydroxylated metabolites, 25-hydroxy vitamin D3 [25-OH D3] and 1a,25-dihydroxy vitamin D3 or calcitriol [1a,25-(OH)2D3], have been found also in different plants[1]. The physiological role of these substances in plants has not been clarified yet but different hypothesis have been considered [2]. Aburjai et al. [3] observed that when calli of Solanum malacoxylon were grown in the light, vitamin D3 and the related hydroxylated metabolites were identified; if the calli were grown in the dark, only 7-dehydrocholesterol was found: these findings suggest that in plant cells may be present a biosynthetic pathway for 1a,25-(OH)2D3 similar to the one acting in animals. To confirm this hypothesis we attempted to identify the enzymes involved in vitamin D3 metabolism in plant. We detected a 25(OH) vitamin D3 1α-hydroxylase activity in mitochondrial membranes purified both from leaves and from cell cultures of Solanum malacoxylon. An activity vitamin D3 25-hydroxylase has also been measured in a reconstituted system containing: a solubilized enzyme fraction obtained from a cell cultures microsomal preparation, NADPH, ferredoxin-NADP+ reductase and ferredoxin I, both recombinant proteins from spinach. An attempt to identify also a 24-hydroxylase, the enzyme responsible for 25-OH D3 and 1a,25-(OH)2D3 catabolism in animals, is in progress.