Sterol carrier protein 2 and fatty acid-binding protein. Separate and distinct physiological functions.

T J Scallen,B J Noland,K L Gavey,N M Bass,R K Ockner,R Chanderbhan,G V Vahouny

doi:10.1016/s0021-9258(18)89132-3

Abstract

Sterol carrier protein 2 (SCP-2) participates in the microsomal conversion of lanosterol to cholesterol, in the conversion of cholesterol to cholesterol ester, and in intracellular cholesterol transfers. The stoichiometry of binding between cholesterol and SCP-2 is 1:1. However, reports have appeared attributing sterol carrier protein activity to a protein preparation identical to hepatic fatty acid-binding protein (FABP). Therefore, the present investigation was conducted to compare homogeneous preparations of FABP and SCP-2 with respect to their capacities to participate as carrier proteins in reactions involving sterols or fatty acids. The results show that SCP-2 and FABP have separate and distinct physiological functions, with SCP-2 participating in reactions involving sterols and FABP participating in reactions involving fatty acid binding and/or transport. Furthermore, there is no overlap in substrate specificities, i.e. FABP does not possess sterol carrier protein activity and SCP-2 does not specifically bind or transport fatty acid. As long as only small quantities of organic solvent (1.6 volume %) were used for substrate addition, the sterol delta 7-reductase liver microsomal assay for SCP-2 correlated well with the physiologically relevant assays employed in the reconstituted adrenal system. The sterol carrier protein activity previously attributed to rat hepatic FABP is explained by the presence of significant quantities of propylene glycol (15 volume %) or Tween 80 in the assay procedure.

Highlights

From the $Department of Biochemistryand Department of Medicine, Schoolof Medicine, Universityof New Mexico, Albuquerque, New Mexico 87131, the TDepartment of Medicine, Schoolof Medicine, University of California, San Francisco, California 94143,and the **Department of Biochemistry, The George Washington University Schoolof Medicine and Health Sciences, Washington, D
The results show that Sterol carrier protein 2 (SCP-2) and FABP have
Reports have appeared attributing sterol carrier protein activity to a protein preparation [9, 10] identical to hepatic FABP [11].There exists, confusion as to the true nature of hepatic sterol carrier protein. Is it SCP-2 or FABP, or do both proteins participate as sterol carriers? the present investigation was conducted to comseparateanddistinct physiological functions, with pare homogenous preparations of FABPandSCP-2 with

Summary

SEPARATE AND DISTINCT PHYSIOLOGICAL FUNCTIONS*

The abbreviations used are: SCP, sterol carrier protein; FABP, lesterol using rat liver microsomes The assay conditions previously described in reports [9, 16] which attributed sterol carrier protein activity to FABP were evaluated with respect to theconcentration of propylene glycol and preincubation procedures as follows, combining the components listed Buffer P (213 pl) and propylene glycol (87 pl) mixed together, NADPH in Buffer P (50 pl), the protein sample to be evaluated (100 p l ) , and 7-dehydrocholesterol.This incubation mixture was preincubated at 37 "C for 15 min. Droplets were reisolated by ultracentrifugation at 4 "C (50,000rpm for 60min), and the cholesterol (Fig. 2 A ) or the arachidonate (Table 111) was measured in the soluble subnatant fraction as previously described [27]

RESULTS AND DISCUSSION

Tyr Phe

Cholesterol formed preincNuobation

Standard assay