The isotopic composition of carbon and nitrogen in individual amino acids isolated from modern and fossil proteins

Abstract

Pigs were reared in laboratory pens on controlled diets that consisted of either 100% C 3 plants or 100% C 4 plants. Carbon and nitrogen isotopic compositions of the diets, and the resulting pig products, purified collagen and muscle tissue, were measured to determine isotopic fractionation during growth and metabolism. Total collagen from pigs grown on C 3 diets was enriched in 13C by 3·2‰ and in 15N by 2·2‰, whereas that from pigs reared on C 4 diets was enriched in 13C by 1·4‰ and in 15N by 2·3‰. In addition, fractionation between pigs and their diets was determined at the molecular level on individual amino acids separated by ion exchange chromatography. The carbon isotopic compositions of separated amino acids from the C 3 and C 4 diets were transferred to amino acids in bone collagen. For nitrogen, the isotopic compositions of all nonessential amino acids were enriched in 15N relative to those amino acids in the diet. Threonine, an essential amino acid, behaved oppositely, in that its isotope ratio (δ 15N) was depleted by an average of 6‰ from the δ 15N of the whole collagen. Similar isotopic patterns were analysed in collagenous amino acids extracted from field specimens that included both herbivores and carnivores; marine animals and terrestrial animals; and C 3 and C 4 feeders. Amino acids from two fossil bones, a bison (4500 years old) and a whale (70,000 years old), recorded the same isotopic signals as modern collagen. The ubiquity of these isotopic patterns at the molecular level suggests that distinct biochemical mechanisms control the metabolism of amino acids in animals rather than random synthesis.