The retention and metabolism of Nτ-methylhistidine by cockerels: implications for the measurement of muscle protein breakdown determined from the excretion of Nτmethylhistidine in excreta

Abstract

1. Excreta were collected for four consecutive days from 4- to 18-week-old cockerels following subcutaneous injection of Nτ-[14CH3]methylhistidine.2. The recoveries of radioactivity in excreta were incomplete and progressively decreased with increasing age.3. Most of the radioactivity not recovered in excreta after 4 d was found in skeletal muscle where > 55% of the radioactivity present was in the Nτ-methylhistidine-containing dipeptide, balenine.4. This peptide appeared to be relatively stable so that most of the labelled Nτ-methylhistidine incorporated was not released during the period of the recovery measurements.5. The total pool of non-protein bound Nτ-methylhistidine (free Nτ-methylhistidine+balenine) in pectoral and mixed thigh muscles increased with age and relative to the daily excretion of Nτ-methylhistidine. At 18 weeks the pool was 3.3 times the daily excretion of Nτ-methylhistidine.6. These observations account for the decreasing recoveries of radioactivity in excreta described previously, due to progressive dilution of labelled Nτ-methylhistidine in an expanding pool of non-protein-bound Nτ-methylhistidine, part of which was relatively stable.7. It is concluded that excretion of Nτ-methylhistidine by 4- to 18-week-old cockerels cannot be used as a reliable index of muscle protein breakdown in vivo.