When compared to Old World primate species, New World primates reared in captivity exhibit an inordinately high incidence of rickets and osteomalacia. The disease is indistinguishable from vitamin D deficiency, but occurs in the presence of extraordinarily high, not low, blood levels of the vitamin D hormone, 1,25-dihydroxyvitamin D3 (l,25-(OH)2D3). This finding infers a state of "resistance" to the hormone. We have discovered that this "resistant state" is not due to a defect in the receptor protein for l,25-(OH)2D3 (VDR) but rather is due to the presence of an intracellular protein (IDBP) that competitively binds l,25-(OH)2D3 and effectively inhibits its ability to bind to the VDR. IDBP also binds other steroids hormones, and it is likely responsible for the "global" nature of steroid resistance in New World primates. The aim of the proposed research is two-fold. The first objective is to purify the IDBP, determine part of its protein sequence, and clone the complementary DNA (cDNA) which codes for the protein. We will accomplish purification of IDBP by a series of chromatographic steps and by protein gel electrophoresis. We will clone the cDNA from a library of cDNAs developed from messenger RNA (mRNA) extracted from the cells of a vitamin D- resistant New World primate. The second objective is to determine the function of this protein in New World primate cells. Using a variety of biochemical, molecular, and cell biology techniques we will investigate how expression IDBP results in the steroid/sterol hormone-resistant phenotype. We anticipate the results of this work will: l) uncover a previously unrecognized layer of control for modulation of cellular steroid hormone-receptor interaction; 2) provide important clues as why most New World species developed and maintained a form of compensated target-tissue resistance to steroid hormones; 3) help us understand new ways in which steroid/sterol hormone metabolism can be altered in primates, including man, to meet the needs of the host; and 4) provide a explanation for human steroid hormone-resistant states not associated with mutations in the steroid receptor genes.