Myosin, 3-methylhistidine

The smdy of tissue protein breakdown in vivo is difficult, because amino acids released during intracellular breakdown of proteins can be extensively reutilized for protein synthesis within the cell, or the amino acids may be transported to other organs where they enter anabohc pathways. However, actin and myosin are methylated by a posttranslational reaction, forming d-methylliistidine. During intracellular breakdown of actin and myosin, 3-methylhistidine is released and excreted into the urine. The urinary output of the methylated amino acid provides a rehable index of the rate of myofibrillar protein breakdown in the musculature of human subjects. 

Modified amino acids, which occur in special proteins such as hydroxyproline in collagen and 3-methylhistidine in actin and myosin, can be used as indicators of the degradation of these proteins. 

Protein ( histidine ) Methyltransferase. An enzyme which methylates histidine in proteins to give primarily 3-methylhistidine residues has been observed in myofibrillar protein and in the sarcoplasmic fraction of muscle homogenates (218). S-Adenosyl-L-methionine serves as the methyl donor for the enzyme. The enzyme has not been solubilized and purified. Very little is known about the substrate specificity of protein-(histidine) methyltransferase. Actins from a wide variety of species consistently contain one 3-N-methylhistidine residue per molecule (191, 219). It appears that myosin from white muscle contains two residues of 3-N-methylhistidine (one residue per heavy chain), whereas myosin from red muscle contains no 3-N-methylhistidine (220). The amino acid sequence around the methylated residue of rabbit skeletal muscle is (221) ... 

The amino acid sequence in rabbit cardiac muscle myosin is homologous with that around the 3-N-methylhistidine residue in rabbit skeletal muscle myosin, yet it does not contain a 3-N-methylhistidine residue. 

The minor amino acid typically present in the meat myofibrillar protein actin (also in some myosin isoforms and in dipeptide anserine, see Section 2.3.3.1.3) is L-3-methylhistidine (2-8), which is formed by methylation of histidine bound at the 73rd position of the protein chain. The functional significance of this modification of actin is not known it is probably related to the metabolism of phosphates, with which the side chain of methylhistidine interacts. Methylhistidine does not occur in other protein-rich foods such as milk, eggs and soybeans. Its contents might therefore serve as a criterion for determining the quality ingredients in meat products. 

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