3-Methylhistidine is a normal constituent of chains of actin and myosin and is formed by post-translational donation of a methionine methyl group to actin and myosin histidine. After degradation of these proteins, the liberated methylated histidine (3-methylhisti-dine) is not reused but quantitatively excreted in the urine of animals. As such, 3-methylhistidine has been used as a method to continuously monitor the breakdown of myofibrillar proteins in muscle (Young and Munro, 1978; Ward and Buttery, 1980; McCarthy et al., 1983). The amount of 3-methylhistidine excreted by an animal is dependent on the myofibrillar protein catabolic rate. Few quantitative data are available on 3-methylhistidine excretion and factors affecting myofibrillar protein catabo-lism in adult cats and dogs. Marks et al. (1996) showed that 3-methylhistine can be used to measure skeletal muscle protein losses or gains based on the quantitative recovery of intravenously administered 3-[14C]methylhistidine. The latter authors reported an average 3-methylhistidine excretion by two adult cats of 5.8 mg 24 It1. In growing dogs, Hill et al. (2001) found that radioactivity of 3-[14C]methylhis-tidine was lost in C02 and/or re-circulated in the body, as reported for sheep and pigs. The latter indicates that 3-methylhistidine cannot be used as a method to measure canine skeletal muscle (myofibrillar) protein breakdown. Rathmacher and Nissen (1998) however described 3-methylhistidine metabolism in dogs with a simple three-compartment model with one urinary exit. The latter authors measured a de novo synthesis of 3-methylhistidine in adult dogs of 12.1 (xmol kg 1 day A lower value of 5.0 (xmol kg 1 day 1 was reported by Hoppe et al. (1993) in adult beagles. Further studies are required to determine the effects of nutrition, hormones, and gender on 3-methylhis-tidine production and the suitability of 3-methylhistidine as a quantitative index of in vivo muscle catabolism in dogs.
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