In the present study, the role of vitamin D in the regulation of estrogen synthesis in gonads was investigated. Vitamin D receptor null mutant mice showed gonadal insufficiencies. Uterine hypoplasia and impaired folliculogenesis were observed in the female, and decreased sperm count and decreased motility with histological abnormality of the testis were observed in the male. The aromatase activities in these mice were low in the ovary, testis, and epididymis at 24%, 58%, and 35% of the wild-type values, respectively. The gene expression of aromatase was also reduced in these organs. Elevated serum levels of LH and FSH revealed hypergonadotropic hypogonadism in these mice. The gene expressions of estrogen receptor alpha and beta were normal in gonads in these mice. Supplementation of estradiol normalized histological abnormality in the male gonads as well as in the female. Calcium supplementation increased aromatase activity and partially corrected the hypogonadism. When the serum calcium concentration was kept in the normal range by supplementation, the aromatase activity in the ovary increased to 60% of the wild-type level, but LH and FSH levels were still elevated. These results indicated that vitamin D is essential for full gonadal function in both sexes. The action of vitamin D on estrogen biosynthesis was partially explained by maintaining calcium homeostasis; however, direct regulation of the expression of the aromatase gene should not be neglected.

Exposure of the skin to sunlight results in an increase of the content of epidermal trans-urocanic acid, a key metabolite of L-histidine, and also in occurrence of the isomerization of trans-urocanic acid to the cis isomer. S-[2-Carboxy-1-(1H-imidazol-4-yl)ethyl]glutathione (GS(CIE)), an adduct of urocanic acid and glutathione, is a presumed origin of a urinary compound S-[2-carboxy-1-(1H-imidazol-4-yl)ethyl]-L-cysteine (Cys(CIE)). The formation of GS(CIE) is stimulated by exposing the skin to sunlight irradiation. In this study we investigated an enzymatic formation of GS(CIE) from glutathione and cis-urocanic acid by incubation with rat liver extract that contained glutathione S-transferase (GST) at high activity. The formation of GS(CIE) was suppressed significantly when a liver extract depleted of GST activity was used. Enzymatic degradation of GS(CIE) with gamma -glutamyl transpeptidase resulted in the formation of N-[S-[2-carboxy-1-(1H-imidazol-4-yl)ethyl]-L-cysteinyl]glycine, a metabolic intermediate between the glutathione adduct and Cys(CIE). A hydrolyzed product of GS(CIE) by HCl was identical with the urinary Cys(CIE). Compounds were analyzed by high-voltage paper electrophoresis, capillary electrophoresis, and fast atom bombardment mass spectrometry. From these results, we suggest that GS(CIE) formed from cis-urocanic acid and glutathione is an origin of the urinary compound Cys(CIE) and that the formation reaction is catalyzed mostly by the action of GST.

Exposure of the skin to sunlight results in an increase in the content of epidermal urocanic acid, a key metabolite of L-histidine, and some portions of the metabolite penetrate into the body fluid. S-[2-Carboxy-1-(1H-imidazol-4-yl)ethyl]glutathione (GS(CIE)), an adduct of glutathione and urocanic acid, was proposed to be an origin of a urinary compound, S-[2-carboxy-1-(1 H-imidazol-4-yl)ethyl]-L-cysteine (Cys(CIE)). Various catabolites of Cys(CIE) were also isolated from human urine previously. However, no direct evidence to show the existence of GS(CIE) as a biological material had been found. By using capillary electrophoresis, the glutathione adduct has now been found in the extracts of rat tissues from the kidney, liver, skin and blood when the rat was kept under conditions of sunlight irradiation after the fur on the dorsal skin had been clipped. On the other hand, no or a trace of GS(CIE) was determined in rat tissue extracts when the animal was kept indoor in usual manner. The glutathione adduct was isolated from the kidney extract of the sunlight-irradiated rat using ion-exchangers and high-voltage paper electrophoresis, and determined by fast-atom-bombardment mass spectrometry. These results indicate that GS(CIE) formation actually occurs in the body and that the formation is accelerated by exposing the rat to sunlight irradiation. From these findings, we propose an alternative pathway of histidine metabolism which is initiated by the adduction of urocanic acid to glutathione to form GS(CIE) and terminates with the formation of the urinary compounds via Cys(CIE).