Abstract
Protein turnover is an energy-consuming process essential for ensuring the maintenance of living organisms. Gold standard methods for protein turnover measurement have inherent drawbacks precluding their generalization for large farm animals and during long periods. We proposed here a non-invasive proxy of the whole-body fractional protein degradation rate (WBPT; whole-body protein turnover for a growing animal) in the long term and in a large number of beef cattle. The proxy is based on the rate at which urine-N and plasma proteins are progressively depleted in 15N after a slight decrease in the isotopic N composition of diet (i.e. diet-switch). We aimed to test the ability of this proxy to adequately discriminate the WBPT of 36 growing-fattening young bulls assigned to different dietary treatments known to impact the protein turnover rate: the protein content (Normal vs High) and amino acid profile (balanced vs unbalanced in methionine). The 15N depletion rate found in plasma proteins represented their fractional synthesis rate, whereas the slowest depletion rate found in urines was interpreted as a proxy of the WBPT. The proxy here tested in urines suggested different WBPT values between Normal vs High protein diets but not between balanced vs unbalanced methionine diets. In contrast, the proxy tested in plasma indicated that both dietary conditions affected the fractional synthesis rate of plasma proteins. We consider that the rate at which urines are progressively 15N-depleted following an isotopic diet-switch could be proposed as a non-invasive proxy of the whole-body protein turnover rate in large farm animals.