P/O ratios reassessed: mitochondrial P/O ratios consistently exceed 1.5 with succinate and 2.5 with NAD-linked substrates.

The efficiency of ATP synthesis coupled to cell respiration, commonly referred to as the P/O ratio, has been the subject of extensive studies for more that 50 years. The general conclusion from these studies is that respiring mitochondria can convert external ADP to ATP at a maximal P/O ratio of 3 for NAD-linked substrates and 2 for succinate. However, in recent years the validity of these "integral" values has been questioned on both mechanistic and thermodynamic grounds, and a mechanistic P/O ratio of 2.5 for NAD-linked substrates and 1.5 for succinate have been concluded on the basis of experiments with isolated mitochondria. These values have been widely adopted in the scientific literature, including several recent textbooks. In this paper we report that under optimal conditions with respect to preparation and assay procedures, the P/O ratios obtained with isolated rat liver mitochondria consistently exceed 2.5 with NAD-linked substrates and 1.5 with succinate. These results, although not excluding "nonintegral" P/O ratios due to various energy-dissipating side reactions, warrant caution in accepting the reported lower values and, in general, in referring to mechanistic considerations unless the underlying molecular mechanisms are understood.