The retention and metabolism of N tau-methylhistidine by cockerels: implications for the measurement of muscle protein breakdown determined from the excretion of N tau-methylhistidine in excreta.
Excreta were collected for four consecutive days from 4- to 18-week-old cockerels following subcutaneous injection of N tau-[14CH3]methylhistidine. The recoveries of radioactivity in excreta were incomplete and progressively decreased with increasing age. Most of the radioactivity not recovered in excreta after 4 d was found in skeletal muscle where greater than 55% of the radioactivity present was in the N tau-methylhistidine-containing dipeptide, balenine. This peptide appeared to be relatively stable so that most of the labelled N tau-methylhistidine incorporated was not released during the period of the recovery measurements. The total pool of non-protein bound N tau-methylhistidine (free (free N tau-methylhistidine + balenine) in pectoral and mixed thigh muscles increased with age and relative to the daily excretion of N tau-methylhistidine. At 18 weeks the pool was 3.3 times the daily excretion of N tau-methylhistidine. These observations account for the decreasing recoveries of radioactivity in excreta described previously, due to progressive dilution of labelled N tau-methylhistidine in an expanding pool of non-protein-bound N tau-methylhistidine, part of which was relatively stable. It is concluded that excretion of N tau-methylhistidine by 4- to 18-week-old cockerels cannot be used as a reliable index of muscle protein breakdown in vivo.
D000367:Age Factors; D000818:Animals; D002645:Chickens / Q000378:metabolism*; D004032:Diet; D004151:Dipeptides / Q000378:metabolism; D005243:Feces / Q000032:analysis; D006639:Histidine / Q000031:analogs & derivatives*; D008297:Male; D008762:Methylhistidines / Q000378:metabolism* / Q000652:urine; D009124:Muscle Proteins / Q000378:metabolism*; D014018:Tissue Distribution
D004151:Dipeptides; D008762:Methylhistidines; D009124:Muscle Proteins; C028119:N(beta)-alanyl-1-methyl-histidine; D006639:Histidine; C028118:3-methylhistidine