LOCALIZATION AND PURIFICATION OF SERUM-ALBUMIN IN THE TESTIS OF XENOPUS-LAEVIS

The distribution of serum albumin is of interest in the Xenopus (X.) l aevis testis, since albumin is probably a major protein that binds tes tosterone (T) in the plasma and interstitial fluid. This study was und ertaken to determine the localization and purification of serum albumi n in the X. laevis testis. The interstitial tissue and spermatogonia i mmunoreacted strongly with a sheep antiserum raised against X. laevis albumin. A weak staining was also seen in spermatocytes and early sper matids, but there was no staining in Sertoli cells. In order to clarif y whether serum albumin was really localized on the surface of testicu lar cells in the X. laevis testis, a membrane-rich fraction was prepar ed from testes and extracted with 0.6 M KCl. The KCl extract was then subjected to gel filtration, ammonium sulfate precipitation and high-p erformance liquid chromatography (HPLC). A protein with Mr=74 kD was o btained by this procedure and its NH2-terminal amino acid sequence was determined. The sequence of the first 19 amino acids was DTDADXXKXIAD VYTALTE, suggesting that this protein was identical to serum albumin ( Mr=74kD). When the membrane fraction of blood cells in this animal was handled in the same manner, no appreciable amount of albumin was dete cted. These results suggest that the 74 kD serum albumin, possibly ass ociated with bound T, may play an important role in the differentiatio n of germ cells during spermatogenesis of X. laevis testis.