Retinol-binding protein is in the molten globule state at low pH

• Published in: Biochemistry (Easton) Vol. 31; no. 33; pp. 7566 - 7571
• Main Authors: Bychkova, V. E, Berni, Rodolfo, Rossi, Gian Luigi, Kutyshenko, V. P, Ptitsyn, O. B
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 25.08.1992
• Subjects: acidic; Analytical, structural and metabolic biochemistry; Apoproteins - chemistry; Apoproteins - metabolism; Binding and carrier proteins; Biological and medical sciences; C.D; Circular Dichroism; fluorescence; Fundamental and applied biological sciences. Psychology; Humans; Hydrogen-Ion Concentration; Magnetic Resonance Spectroscopy; N.M.R; Osmolar Concentration; Protein Conformation; Proteins; retinol-binding protein; Retinol-Binding Proteins - chemistry; Retinol-Binding Proteins - metabolism; Spectrometry, Fluorescence; Spectrophotometry, Ultraviolet; Thermodynamics; Vitamin A - metabolism; Human; Binding protein; Apoproteins; TBPA; pH; Environmental factor; Secondary structure; Spatial structure; Molecular dissociation; Retinol; Conformational transition
• ISSN: 0006-2960; 1520-4995
• DOI: 10.1021/bi00148a018

Using far- and near-UV circular dichroism, viscosity, tryptophan fluorescence, NMR spectra, binding of a hydrophobic probe, and microcalorimetry, we have shown that the apo form of human retinol-binding protein (RBP) at neutral pH is in a rigid state with properties similar to those of holo-RBP. On the contrary, at acidic pH apo-RBP is in the molten globule state which has been earlier revealed for a number of proteins under mild denaturing conditions. We have also shown that, at equilibrium, the pH-induced retinol release from holo-RBP parallels denaturation of the apoprotein. These findings are consistent with our hypothesis that the transformation of RBP into the molten globule state is involved in the mechanism whereby retinol is delivered to target cells. In particular, a local acidic pH near the membrane surface of target cells might cause the transition of RBP to the molten globule state as well as the release of retinol.