Extraction of brain capillary membrane proteins using Triton X-114. 
Brain capillaries contain a great variety of membrane proteins involved in the transport of hydrophilic nutrients or in the reception of hormonal signals.  The use of Triton X-114 fractionation to purify membrane proteins according to their degree of hydrophobicity was investigated.  Analysis by polyacrylamide gel electrophoresis showed a distinct polypeptide composition for each fraction.  Most of the proteins (68%) were solubilized by Triton X-114 and, of these proteins, the majority (74%) was found in the detergent-poor phase.  Alkaline phosphatase which possesses a glycosyl-phosphatidylinositol anchor partitioned in the pellet of insoluble proteins where it was enriched 2.3-fold.  In contrast, gamma-glutamyltranspeptidase, the GLUT1 glucose transporter and P-glycoprotein, three integral membrane proteins, and p21ras and a 42 kDa G protein alpha subunit, both covalently modified by lipids, were efficiently solubilized and fractionated in the detergent-rich fraction where they were enriched 3.5-, 4.8-, 4.4-, 4.5- and 4.7-fold, respectively.  Triton X-114 fractionation could therefore be used as a first step in the purification of many blood-brain barrier membrane proteins.