Mitochondrial respiratory function in a patient with maternally inherited type 2 diabetes mellitus and hypertrophic cardiomyopathy associated with heteroplasmic mitochondrial DNA (mtDNA) C3310T mutation, which replaces the second amino acid of NADH dehydrogenase 1 (ND1) from a hydrophobic Proline to a hydrophilic Serine, was investigated. Mitochondrial respiratory function solely due to mtDNA C3310T mutation was investigated in cybrid system by the fusion of mtDNA-deleted (rho(0)) HeLa cells and exogenous mtDNA either from the proband or from controls. Total oxygen consumption of the proband cybrid cells was significantly decreased compared with those of controls (2.468+/-0.475 versus 2.871+/-0.484 micromol/h/10(7) cells, p=0.0392). Mitochondrial respiratory chain complex I activity of the proband cybrid cells was also significantly decreased compared with those of controls (0.191+/-0.080 versus 0.288+/-0.113 micromol/h/mg protein, p=0.0223). Furthermore, ATP content in the proband cybrid cells was also significantly decreased compared with those in controls (1.119+/-0.344 versus 1.419+/-0.378 pmol/10(5) cells, p=0.044). The present study indicates that mtDNA C3310T mutation may be a pathogenic mutation of maternally inherited type 2 diabetes mellitus and hypertrophic cardiomyopathy in the proband and the family.