Rat testicular testibumin is a protein responsive to follicle stimulating hormone and testosterone that shares immunodeterminants with albumin.

During a search for hormonally responsive products in media from Sertoli cell enriched cultures, a major follicle stimulating hormone responsive and testosterone-responsive protein was identified and designated CMB-1. The results of the present study indicate that this protein is related immunologically to rat albumin and rat alpha-fetoprotein (AFP) and is concentrated in the testis of the adult rat. CMB-1 was therefore termed testibumin. Testibumin was purified from Sertoli cell enriched cultures to apparent homogeneity by sequential high-performance liquid chromatography on anion-exchange, chromatofocusing, gel permeation, and hydroxylapatite columns. The purified protein consists of two concanavalin A (Con A) reactive forms: one which does not interact with Con A and the other which binds to this lectin and is eluted with methyl alpha-mannoside. Testibumin is a monomer with an apparent molecular weight of 69,000 and a pI ranging between 4.5 and 4.85. The heterogeneity of this protein was further demonstrated by crossed-immunoelectrophoresis and two-dimensional gel electrophoresis. A monospecific antiserum and highly purified testibumin were used to develop a specific radioimmunoassay which permitted studies of the hormonal responsiveness of Sertoli cell enriched culture and of the content of testibumin in the reproductive tract fluids in vivo. Even though testibumin was found in serum of both sexes, it was highly concentrated in the testicular and epididymal compartments in adult rats. This protein was compared to rat serum albumin and rat AFP immunologically. With the use of immunoblots, antiserum developed against testibumin showed partial cross-reactivity with albumin and AFP when these latter proteins were denatured and were present in amounts several orders of magnitude greater than testibumin. The extent of this cross-reactivity was then examined by comparing the ability of native and S-carboxymethylated albumin to compete with 125I-testibumin for binding to a monospecific testibumin antiserum. It was shown that the unfolded derivative of albumin showed partial cross-reactivity with testibumin. We conclude that testibumin is immunologically related to albumin and AFP as these latter proteins are related to one another and that testibumin is possibly the homologue of albumin in the seminiferous tubular compartment.