The urinary excretion of N-methyl histidine by cattle: validation as an index of muscle protein breakdown

Abstract

1. The recoveries of radioactivity in cattle urine following the intravenous administration of N^τ-[¹⁴CH₃]methyl histidine were essentially quantitative in 5–7 d in non-lactating cows, bulls and steers and did not change with age.2. The N^τ-methyl histidine was excreted unchanged in urine.3. N^τ-methyl histidine occurred in muscle extracts both in the free form and as a perchloric acid-soluble, acid-labile form which accounted for approximately 85% of the total non-protein-bound N^τ-methyl histidine in muscle and appeared identical to a similar component identified in muscle extracts of sheep and pigs.4. There was probably an age-related decrease in the concentration of the acid-labile component in muscle but which did not produce a measurable change in recovery of radioactivity in urine.5. The daily excretion of N^τ-methyl histidine (E, μmol) by male cattle was highly correlated with live weight (W, kg) by the equation: E=50⋅4+3⋅536(±0⋅044)W(r0⋅997). The excretions progressively decreased from 4.04 μmol/d per kg at 100 kg weight to 3.62 μmol/d per kg at 600 kg.6. By the criterion of the rate of clearance of labelled N^τ-methyl histidine from the body, the excretion of N^τ-methyl histidine in urine appears to be a valid index of muscle protein breakdown in cattle.