Metabolism of [3 alpha-H-3] 25-hydroxyvitamin D-2 in kidneys isolated fromnormal and vitamin D-2-intoxicated rats

With the availability of A-ring labelled 25OHD(2), [3 alpha-H-3] 25OHD(2), we have performed the present study to examine the metabolism of 25OHD(2) u sing physiological substrate concentrations in perfused kidneys isolated fr om both normal and vitamin D-2-intoxicated rats. Our results indicate that [3 alpha-H-3] 25OHD(2) is metabolized into both 24(S),25,28-trihydroxyvitam in D-2 [24(S),25,28(OH)(3)D-2] and 24(R),25,26-trihydroxyvitamin D-2 [24(R) , 25,26(OH)(3)D-2], and the amounts of these two metabolites produced in th e kidney of vitamin D-2-intoxicated rat were about 3-5 times higher than th ose produced in the kidney of normal rat, Similar results were also obtaine d with rat kidney homogenates incubated with [3 alpha-H-3] 25OHD(2). Furthe rmore, we noted that the production of both 24(S),25,28(OH)(3)D-2 and 24(R) ,25,26(OH)(3)D-2 in the kidney homogenates of vitamin D-2-intoxicated rats increased with the time of incubation and then subsequently decreased, The decrease in both 24(S),25,28(OH)(3)D-2 and 24(R),25,26(OH)(3)D-2 coincided with an increase in the fraction of total radioactivity distributed in the aqueous phase of the kidney homogenates. This finding suggested the possibi lity of further metabolism of 24(S),25,28(OH)(3)D-2 and 24(R), 25,26(OH)(3) D-2 into polar water-soluble metabolite(s). We then measured the radioactiv ity in the aqueous phase of kidney homogenates of both normal and vitamin D -2-intoxicated rats incubated with [3 alpha-H-3] 25OHD(2). It was noted tha t the amount of radioactivity in the aqueous phase of kidney homogenates of vitamin D-2-intoxicated rats is higher than that present in the aqueous ph ase of kidney homogenates of normal rats, Thus, our study provides evidence for the first time for the formation of both 24(S),25,28(OH)(3)D-2 and 24( R),25, 26(OH)(3)D-2 under physiological conditions, and the possibility of their further metabolism into as yet unidentified polar water-soluble metab olite(s). As the formation of all these metabolites is increased in the kid ney of vitamin D-2-intoxicated rats when compared to normal rats, it appear s that the increased rate of metabolism of 25OHD(2) during hypervitaminosis D-2 plays a significant role in the deactivation of 25OHD(2).