Lipid-protein ratios by infrared spectroscopy.

A method for measuring the ratio of diacyl phospholipid to protein in lipid-protein mixtures and membranes by infrared spectroscopy is described. Samples made of diacyl phospholipid and proteins mixed in known ratios were analyzed for lipid-protein ratio by the infrared (ir) method. Results had a standard deviation of less than +/- 4% over the lipid-protein molar ratio range of 9:1 to 320:1. Calculations of the ratio of total lipid to protein require that the diacyl phospholipid-to-protein ratio be divided by the mole fraction of diacyl phospholipid in the total lipid. Phospholipid-protein ratios for various sarcoplasmic reticulum membrane preparations (R1-washed, octylglucoside purified, deoxycholate treated) were determined by the ir method and compared to literature values. Also, phospholipid-protein ratios were determined for R1-washed sarcoplasmic reticulum by three chemical analyses using different protein assays and were compared with ratios obtained by the infrared method. The infrared results were closest to those of a chemical method designed specifically for membrane proteins.