In vivo dihydrotachysterol2 metabolism in normal man: 1 alpha- and 1 beta-hydroxylation of 25-hydroxydihydrotachysterol2 and effects on plasma parathyroid hormone and 1 alpha,25-dihydroxyvitamin D3 concentrations.
It has recently been shown that in the rat, dihydrotachysterol (DHT) is extensively metabolized in the side-chain in vivo along pathways similar to those of vitamin D. In addition 25-hydroxy-DHT2 [25OHDHT2] is hydroxylated at C1, producing both 1 alpha- and 1 beta- hydroxy compounds. An in vivo study in 1988 demonstrated that in normal adult subjects receiving oral DHT2, plasma 1 alpha,25-dihydroxyvitamin D [1,25-(OH)2D] concentrations fell, but with unchanged plasma PTH levels. Down-regulation of 1,25-(OH)2D3 production by 25-(OH)DHT2 or some other unknown metabolite was also suggested as an explanation for these observations. To investigate whether either of the newly characterized 1 alpha,25- or 1 beta,25-(OH)2DHT2 was formed in vivo in normal man, DHT2 (approximately 1 mg/day, orally) was administered to healthy volunteers (three males and one female). Plasma was analyzed by high performance liquid chromatography and gas chromatography-mass spectrometry, demonstrating the formation of both 1 alpha,25- and 1 beta,25-(OH)2DHT2 in vivo in normal human subjects. Plasma levels of 1,25-(OH)2D3, PTH, ionized and total calcium, inorganic phosphate, and alkaline phosphatase were monitored. The plasma concentrations of DHT2, 25OHDHT2, and 1 alpha,25- and 1 beta,25-(OH)2DHT2 were measured by gas chromatography-mass spectrometry. In all volunteers, plasma ionized calcium increased slightly during DHT2 administration; 1,25-(OH)2D3 and PTH concentrations fell. Plasma levels of DHT2 and its metabolites rose over the same period. The average fall in the level of plasma 1,25-(OH)2D (60-70 pmol/L) was mirrored by a rise in the concentration of 1 alpha,25-(OH)2DHT2 (550 pmol/L). This ratio is appropriate, because it has previously been shown that in a reconstituted COS cell, 1 alpha,25-(OH)2DHT3 has roughly one tenth the potency of 1,25-(OH)2D3. At maximum concentration, the ratios of DHT2/25OHDHT2/1 beta,25-(OH)2DHT2/1 alpha,25-(OH)2DHT2 were approximately 10:1:2:0.1. The concentration of 1 beta,25-(OH)2DHT2 was greater than that of 25OHDHT2, and the ratio of 1 alpha,25- to 1 beta,25-(OH)2DHT2 (1:20) was substantially lower than that in rat plasma (3:10). The data presented here suggest that the active DHT2 metabolite in man is 1 alpha,25-(OH)2DHT2 and that the fall in plasma 1,25-(OH)2D seen during DHT therapy may be partly the result of suppressed PTH secretion.
Schroeder
N J
NJ
Department of Chemical Pathology, London Hospital Medical College, United Kingdom.
Trafford
D J
DJ
Cunningham
J
J
Jones
G
G
Makin
H L
HL
D000284:Administration, Oral; D001711:Biotransformation; D002117:Calcitriol / Q000097:blood*; D002118:Calcium / Q000097:blood; D002851:Chromatography, High Pressure Liquid; D004097:Dihydrotachysterol / Q000031:analogs & derivatives* / Q000097:blood / Q000378:metabolism* / Q000494:pharmacology*; D005260:Female; D008401:Gas Chromatography-Mass Spectrometry; D006801:Humans; D006900:Hydroxylation; D008297:Male; D013058:Mass Spectrometry; D010281:Parathyroid Hormone / Q000097:blood*; D012016:Reference Values
D010281:Parathyroid Hormone; C077302:25-hydroxydihydrotachysterol(2); D002117:Calcitriol; D004097:Dihydrotachysterol; D002118:Calcium