Heats of Combustion of Leaf Proteins, and Incidentally of Linseed Mucilage and Citrus Pectin
British Journal of Nutrition
In computing the energy metabolism of animals on various diets it is commonly assumed that the catabolized protein would yield 5-7 Cal.+/g. if burned in such a way that the elements would yield CO" water, H,SO, and nitrogen. This value is based upon the heats of combustion of several proteins of animal origin and numerous proteins obtained from plant seeds. Due allowance must be made for the fact that much of the protein N will ordinarily appear after catabolism as urea N, and hence the net
... rific value to the animal is less than that found by oxidation in a bomb or oxycalorimeter. The precision attainable in animal metabolism studies is quite high with modem equipment, but interpretation of the data is often rendered less certain than it should be by lack of information about the heats of combustion of the particular proteins involved in the metabolism. This is particularly true for diets containing much pasture protein, as the literature provides no information about the heats of combustion of these proteins. This article reports a study of the heats of combustion of four leaf-protein preparations, which were unavoidably contaminated with impurities. It was accordingly necessary to estimate the amounts of impurities pkesent, and their contributions to the heats of combustion. From these data the heats of combustion of the hypothetically 'pure' proteins have been computed. Although allowance has been made for the contribution of these contaminants to the total heat production, the precise nature of the major contaminants in preparations of this kind is unknown. The question has been reviewed by Lugg (1939), who considered that they were probably of the nature of mucilage or pectin. The protein preparations These were the preparations described by Lugg & Weller (1944). Information concerning them is given in Table I . Of the nitrogen present in these preparations about 36% in each case has been accounted for as amide or specific amino-acid in the analyses thus far completed.