Case Report: Congenital methemoglobinemia in a cat with the reduced NADH-cytochrome b5 reductase 3 activity and missense mutations in CYB5R3

A one-year-old castrated male mixed-breed cat was referred for detailed examination of long-term pale mucous membrane without any clinical episodes that may cause cyanosis. While no causative abnormalities were detected in thoracic radiography and echocardiography, and. arterial partial pressure of oxygen was within normal value, methemoglobin concentration of the cat was increased to 30% and cytochrome b5 reductase activity, which converts methemoglobin to hemoglobin, was reduced to 2.0 IU/gHb (13.4 +/- 1.7 IU/gHb in control cats, n = 3), indicating congenital methemoglobinemia. Sequence analysis of CYB5R3, which codes cytochrome b5 reductase, showed two missense mutations found in the patient, one of which was predicted to affect protein function.