Characterization of Plasma Membrane Proteins from Lactating Bovine Mammary Gland

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Properties of proteins in the light (<d 1.03 on Ficoll and then <d 1.14 on sucrose) and heavy (d 1.03/1.05 on Ficoll and then <d 1.14 on sucrose) plasma membranes (PM) isolated from lactating bovine mammary gland were investigated. The PMs consisted of 57-62% of protein and 38-43% lipid. The lipid/protein ratio was 0.77 in the light PM and higher than 0.61 of the heavy PM. However, no appreciable differences were found between the light and heavy PMs with respect to polypeptide, amino acid, and carbohydrate compositions. The protein moiety contained approximate 5-6% carbohydrate: fucose 8-9, mannose 11-12, galactose 16, N-acetylglucosamine 32, N-acetylgalactosamine 12-15, and sialic acid 17-20 mol%. PM protein was high in the content of aspartic acid, glutamic acid, and leucine and low in proiine, cystine, methionine, and histidine. On double immunodiffusion both PMs formed precipitin lines against milk fat globule membrane (MFGM) antiserum. Electrophoretic analysis on sodium dodecyl sulfate-polyacrylamide gel revealed the presence of many minor polypeptides and three major glycopeptides (PAS-I, II, and III) of molecular weights of 115, 000, 94, 000, and 82, 000. The glycoprotein profiles of the PMs were different from those of MFGM, except that PAS-II and -III of PM corresponded to PAS-3 and -4 of MFGM, respectively.
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