The inadequacy of urinary N τ -methyl histidine excretion in the pig as a measure of muscle protein breakdown

Abstract

1. The validity of the urinary excretion of N τ -methyl histidine (N τ -MH) by pigs as an index of muscle protein breakdown in vivo was tested using the criterion of the rate of recovery of radioactivity in urine following an intravenous dose of N τ -[ 14 CH 3 ]methyl histidine. 2. Urinary recoveries of radioactivity from five animals were less than 21% of dose in 7 d after which the daily recovery was less than 0.3% per day. 3. The incomplete recoveries of radioactivity were associated with the presence in muscle of a large. pool of non-protein-bound N τ -MH, the concentration of which increased with age. 4. The N τ -MH in this pool was present as free N τ -MH and in a dipeptide which constituted more than 90% of the total non-protein-bound N τ -MH. The contribution of the peptide increased with age, reaching 99.8% in older animals. 5. The pool of non-protein-bound N τ -MH was maintained and increased in both established and newly accreted tissue by retention of some of the N τ -MH released by muscle protein breakdown, only a proportion of which was therefore available for excretion. Hence, the urinary excretion of N τ -MH is not a valid index of muscle protein breakdown in pig.