1-Methylhistidine, also known as 1-MHis, 1MH, tau-methylhistidine or tele-methylhistidine, belongs to the class of organic compounds known as histidine and derivatives. 1MH is also classified as a methylamino acid. Methylamino acids are primarily proteogenic amino acids (found in proteins) which have been methylated (in situ) on their side chains by various methyltransferase enzymes. Histidine can be methylated at either the N1 or N3 position of its imidazole ring, yielding the isomers 1-methylhistidine (1MH; also referred to as tau-methylhistidine, according to IUPAC) or 3-methylhistidine (3MH; pi-methylhistidine, according to IUPAC), respectively. There is considerable confusion with regard to the nomenclature of the methylated nitrogen atoms on the imidazole ring of histidine in histidine-containing proteins (such as actin and myosin) as well as histidine-containing peptides (such as anserine and ophidine/balenine). In particular, older literature (mostly prior to the year 2000) as well as most biochemists and nutrition scientists incorrectly number the imidazole nitrogen atom most proximal to the side chain beta-carbon as 1 or N1, while organic chemists correctly designate it as 3 or N3. As a result, biochemists and nutrition scientists historically designated anserine (Npi-methylated) as beta-alanyl-N1-methylhistidine (or beta-alanyl-1-methylhistidine), whereas according to standard IUPAC nomenclature, anserine is correctly named as beta-alanyl-N3-methylhistidine. As a result, for several decades, many papers incorrectly identified 1MH as a specific marker for dietary consumption or various pathophysiological effects when they really are referring to 3MH – and vice versa. 1MH can only be generated from histidine residues through the action of methyltransferases as a protein post-translational modification event. Histidine methylation on the 1- or tau site of histidine-containing proteins is mediated by at least two enzymes: SETD3 and METTL18. SETD3, or SET domain-containing protein 3, is a protein-histidine N-methyltransferase that specifically mediates 1-methylhistidine (tau-methylhistidine) methylation of actin at 'His-73'. SETD3 is a methyltransferase that uses S-adenosyl-L-methionine to transfer the methyl group to histidine at the tau position. Histidine methylation of actin His-73 is required for smooth muscle contraction of the laboring uterus during delivery. It also reduces the nucleotide exchange rate on actin monomers and modestly accelerates actin filament assembly. SETD3-mediated histidine methylation appears to occur in all higher eukaryotes with actin, from plants to insects to vertebrates. Within cells, SETD3 is found in the cytoplasm and nucleus. Other proteins that are known to have 1MH modifications include myosin and myosin kinase. In addition to these tau-His-methylated proteins, a specialized dipeptide called ophidine (balenine) that consists of beta-alanine and 1MH is also known. Because 1MH is so abundant in skeletal muscle tissues (being found in the main myofibrillar proteins actin and myosin), the urinary concentrations of 1-methylhistidine can be used as a biomarker for skeletal muscle protein breakdown, especially for those who have been subject to muscle injury. During protein catabolism, 1-methylhistidine is released but cannot be reutilized. Therefore, the plasma concentration and urine excretion of 1-methylhistidine serve as sensitive markers of myofibrillar protein degradation. Approximately 75% of 1-methylhistidine in the human body is estimated to originate from skeletal muscle (3MH/1MH switch - PMID: 32235743 ). In addition to the degradation of muscle proteins, the 1-methylhistidine level can be moderately affected by the degradation of intestinal proteins and meat intake. 1-Methylhistidine has been found to be associated with several diseases such as Alzheimer's disease, preeclampsia, obesity, kidney disease. The normal concentration of 1-methylhistidine in the urine of healthy adult humans has been detected and quantified in a range of 17.7-153.8 micromoles per millimole (umol/mmol) of creatinine, with most studies reporting the average urinary concentration between 25-40 umol/mmol of creatinine. The average concentration of 1-methylhistidine in human blood plasma has been detected and quantified at 12.7 micromolar (uM) with a range of 9.8-15.6 uM. As a general rule, urinary 3MH is associated with white meat intake (p< 0.001), whereas urinary 1MH is associated with red meat intake (p< 0.001).

Metabolism and Physiological Effects of 1-Methylhistidine References