Evidence for the presence of 5'-deiodinase in mammalian seminal plasma and for the increase in enzyme activity in the prepubertal testis.
Thyroid hormones are critical for structural and functional development of the testis and Sertoli cells are considered true target cells for triiodothyronine (T3). However, the role of thyroid hormones in the adult testis seems to be minimal and the mechanism by which they affect testicular function is not known. Due to the existing blood-testis barrier the concentration of thyroid hormones in seminal plasma is kept lower than in blood plasma. We have found that T3 may reach the testis not only from the circulation but also from local enzymic conversion of thyroxine to T3. The presence of the enzymic activity responsible for thyroxine 5'deiodination and for generating T3 locally was also found in boar's seminal plasma. The seminal plasma 5'-deiodinase (5'-D) appeared to be predominantly the propylthiouracil (PTU)-insensitive type II isoenzyme found, so far, in tissues where it plays a role in paracrine signalling. It contains selenocysteine in its molecule (inhibition by aurothioglucose), and has an apparent Km for reverse-T3 as substrate of 0.36 nM and a Vmax 23.8 fmol I-/mg protein/min. Because the seminal plasma 5'-D is partially, but uncompetitively, inhibited by PTU, the presence in seminal plasma of two 5'-D isoenzymes (type I and II) cannot be excluded. The 5'-D activity in testes increased significantly between week 3 and 4, and this increase was concomitant with increase in testicular size. The relationship between testicular weight gain and age showed a similar characteristic change and corresponded to the change in 5'-D activity. Unlike in rodents, the testis of the prepubertal pig has thyroid hormone receptors in Sertoli cells, and suggests that in growing piglets, testicular 5'-D is a key factor regulating local supply of biologically active T3, and is an essential factor in testicular paracrine function. The present results are the first demonstration and characterization of the 5'-deiodinase in seminal plasma.
Brzezińska-Slebodzińska
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E
Department of Developmental and Experimental Endocrinology, Institute of Animal Reproduction and Food Research, Polish Academy of Sciences, ul. Grunwaldzka 250, 60-166 Poznań, Poland. endocrin@rose.man.poznan.pl
Slebodziński
A B
AB
Kowalska
K
K
D000818:Animals; D007453:Iodide Peroxidase / Q000378:metabolism*; D008297:Male; D008322:Mammals; D012661:Semen / Q000201:enzymology*; D012741:Sexual Maturation / Q000502:physiology*; D013552:Swine; D013737:Testis / Q000201:enzymology*
D007453:Iodide Peroxidase