Sterol Binding Research Articles

Specificity and Role in Cholesterol Biosynthesis of a Squalene and Sterol Carrier Protein

Abstract A purified (300-fold) liver protein specifically binds squalene and sterol precursors of cholesterol and activates (4-fold or greater) the microsomal enzymic steps of cholesterol biosynthesis in which these precursors participate. This heat-stable protein appears to play a general role as vehicle for cholesterol and its water-insoluble precursors, i.e. it is a squalene and sterol carrier protein (SCP). Apo-SCP (molecular weight 16,000) aggregates to a higher molecular weight species (g150,000) during binding of squalene or sterols and yields a noncovalent squalene- or sterol-SCP complex. Apo-SCP similarly binds stoichiometric levels of a cholesterol precursor and the pyridine nucleotide cofactor required for the oxidation or reduction of the precursor by microsomal enzymes. The apparent Km for a particular sterol-SCP complex is markedly lower than that for initially unbound sterol, although the maximum rate of the particular reaction is unchanged. Of the other liver and serum proteins tested, only apo-high density lipoprotein (HDL) will substitute for the binding and activation functions of apo-SCP. A component of HDL may be identical with apo-SCP.

Binding of antibiotics to tissue homogenates.

Binding of metabolites and drugs, including antibiotics, to serum proteins has been extensively studied. This phenomenon has important biologic and therapeutic significance [1-3]. The ability of tissues to bind has also been explored, particularly with organic dyes and biologically active agents. In some instances, binding of drugs to specific sites in microorganisms can be linked closely with their mode of action. For example, the polymyxins owe their biologic activity to binding to polyphosphate groups of phospholipids in the bacterial cell membrane [4], and amphotericin B requires sterol binding sites in fungal cells [5]. Antitrypanosomal activity of acriflavine has been shown to depend upon binding of the drug to the parasite [6]. The receptor site theories of pharmacology and virology depend largely on evidence of specific combination between drug or virus and complementary structures on cell membranes. Some of these concepts of drug action were clearly visualized by Ehrlich and Morgenroth many years ago [7 p. 24]. Studies in this laboratory indicated that several antibiotics, particularly those in the polymyxin and aminoglycoside families, are inhibited by constituents of bacteriologic agar and various culture media [8]. The phenomenon appears to be related to the charge produced by free amino groups in the drugs and sulfate groups in the medium, since it can be overcome readily by addi-

The binding of sterols in cellular membranes.

The binding of sterols in cellular membranes.