The stimulatory effect of albumin on luteinizing hormone-stimulated Leydig cell steroid production depends on its fatty acid content and correlates with conformational changes

__Abstract__ The effects of purified albumin species and albumin fragments (0.2–1% w/v) on short-term (4 h) steroid secretion by immature rat Leydig cells, in the presence of a maximally stimulating dose of luteinizing hormone (LH), were investigated. Human albumin and the peptic fragment (comprising residues 1–387) enhanced pregnenolone production in isolated rat Leydig cells, whereas chicken albumin and the tryptic fragment (comprising residues 198–585) were not active. This stimulatory effect of human albumin and the peptic fragment correlated with the potential of these proteins to undergo a pH-dependent neutral-to-base transition as measured by circular dichroism. The tryptic fragment and chicken albumin did not have the potential to undergo such a transition. The pH-dependent conformational changes of albumin and fragments thereof occurred in parallel with a change in the binding affinity for testosterone and pregnenolone. The fatty acid oleic acid and the drug suramin, only when present in a molar ligand-to-albumin ratio equal to or higher than 2, inhibited the albumin-mediated stimulation of steroid production. These data show that the stimulatory effects of albumin species on LH-induced Leydig cell pregnenolone production depend on their fatty acid content and correlate with the potential of these molecules to undergo conformational changes. It is unknown via which mechanisms albumin exerts its stimulatory effect, but the LH action through the cyclic AMP pathway seems not to be affected

Location and characterization of the warfarin binding site of human serum albumin A comparative study of two large fragments

The warfarin binding behaviour of a large tryptic fragment (residues 198–585 which comprise domains two and three) and of a large peptic fragment (residues 1–387 which comprise domains one and two) of human serum albumin has been studied by circular dichroism and equilibrium dialysis in order to locate and characterize the primary warfarin binding site. The induced ellipticity of the warfarin-peptic fragment complex turned out to be pH dependent. This pH dependence occurs in the region of the neutral-to-base transition of the albumin molecule. The induced ellipticity of the warfarin-tryptic fragment complex is pH independent. Difference CD-spectra showed that the peptic fragment and albumin have similar warfarin binding properties whereas the tryptic fragment has deviant warfarin binding properties. The equilibrium dialysis experiments showed that the affinity of warfarin to the peptic fragment and to albumin is practically the same whereas the affinity of warfarin to the tryptic fragment is a factor 2–8 lower than the affinity of warfarin to albumin. Our results indicate that the main part of the primary warfarin binding site is located in domain two of the albumin structure and that domain one plays an important role in the N-B transition of albumin