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 studie s for more than 50 years. The general conclusion from these studies is that respiring mitochondria can convert external ADP to ATP at a maxi mal P/O ratio of 3 for NAD-linked substrates and 2 for succinate. Howe ver, in recent years the validity of these ''integral'' values has bee n questioned on both mechanistic and thermodynamic grounds, and a mech anistic P/O ratio of 2.5 for NAD-linked substrates and 1.5 for succina te have been concluded on the basis of experiments with isolated mitoc hondria. These values have been widely adopted in the scientific liter ature, including several recent textbooks. In this paper we report tha t under optimal conditions with respect to preparation and assay proce dures, the P/O ratios obtained with isolated rat liver mitochondria co nsistently exceed 2.5 with NAD-linked substrates and 1.5 with succinat e. These results, although not excluding ''nonintegral'' P/O ratios du e to various energy-dissipating side reactions, warrant caution in acc epting the reported lower values and, in general, in referring to mech anistic considerations unless the underlying molecular mechanisms are understood.