Hemoglobin derivatives are often quantified in blood to establish cardio-respiratory status and possible causes of impaired oxygen transport. The derivative known as methemoglobin results from oxidation of hemoglobin and is pathologically relevant because it cannot transport oxygen. In species and individuals possessing unstable methemoglobin, methemoglobin formation leads to rapid hemichrome formation and precipitation. Oxidizing reagents in standard methemoglobin analysis techniques therefore prevent accurate quantification of hemoglobin oxidative degradation products in species possessing unstable hemoglobin. In this study, we demonstrated that individual coho salmon (Oncorhynchus kisutch) possess unstable methemoglobin. Because molar absorptivities of coho methemoglobin, hemichrome and carboxyhemoglobin were significantly different from humans, the use of previous standard methods leads to an overestimation of methemoglobin in coho. Spontaneous conversion of methemoglobin to hemichrome was also demonstrated in Chinook (O. tshawytscha), pink (O. gorbuscha) and chum salmon (O. keta), but not steelhead (O. mykiss), indicating there may be a frequent need to account for unstable hemoglobin when quantifying methemoglobin in salmonids. • Our method builds upon multi-component analysis (MCA) by using a multivariate modeling technique to derive the coho-specific molar absorptivities of major hemoglobin derivatives • This approach fills a current need for the accurate quantification of methemoglobin in fishes possessing unstable hemoglobin
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