Presence of complex V subcomplexes in the patients with oxphos defects.

The oxidative phosphorylation (OXPHOS) generates ATP through passage of electrons along the four complexes of the respiratory chain (I-IV) in the mitochondrial inner membrane. ATP synthase (complex V) is a multi-subunit enzyme consisting of a catalytic portion (F1), a membrane portion (F0) and two stalks linking F1 and F0. Using Blue Native Polyacrylamide Gel Electrophoresis and subsequent catalytic staining for ATPase, only one band corresponding to complex V was detected in most of the patients. In nine patients, however, in addition to the normal band, complex V subcomplexes with lower molecular weight were detected. This phenomenon was seen in skeletal muscle (2 patients), liver (3 patients) and in cultured skin fibroblasts (4 patients). A severe, combined deficiency of OXPHOS complexes (I, III, IV and V) together with normal activity of complex II was seen in two patients, and a less severe, combined deficiency (I and IV) in four patients. Two of the latter patients were carriers of a MERRF point mutation. In three patients no other defects of OXPHOS complexes were found. The exact cause of incomplete complex V synthesis, or decreased stability is not known. One likely hypothesis is a defective intramitochondrial protein synthesis. The associated finding of a combined OXPHOS complex deficiency in several patients is in favor of this hypothesis. Remarkably, two of these patients carried a MERRF mutation. Other possible explanations are a mutation in a complex V subunit, especially in subunit 6 (Nijtmans et al., 2001; Houstek et al., 1995), or increased intramitochondrial oxidative stress (Guerrieri et al., 1999).