Localization and purification of serum albumin in the testis of Xenopus laevis.

The distribution of serum albumin is of interest in the Xenopus (X.) laevis testis, since albumin is probably a major protein that binds testosterone (T) in the plasma and interstitial fluid. This study was undertaken to determine the localization and purification of serum albumin in the X. laevis testis. The interstitial tissue and spermatogenia immunoreacted strongly with a sheep antiserum raised against X. laevis albumin. A weak staining was also seen in spermatocytes and early spermatids, but there was no staining in Sertoli cells. In order to clarify whether serum albumin was really localized on the surface of testicular cells in the X. laevis testis, a membrane-rich fraction was prepared from testes and extracted with 0.6 M KCl. The KCl extract was then subjected to gel filtration, ammonium sulfate precipitation and high-performance liquid chromatography (HPLC). A protein with M(r) = 74 kD was obtained by this procedure and its NH2-terminal amino acid sequence was determined. The sequence of the first 19 amino acids was DTDADXXKXIADVYTALTE, suggesting that this protein was identical to serum albumin (M(r) = 74 kD). When the membrane fraction of blood cells in this animal was handled in the same manner, no appreciable amount of albumin was detected. These results suggest that the 74 kD serum albumin, possibly associated with bound T, may play an important role in the differentiation of germ cells during spermatogenesis of X. laevis testis.