Kidney-cortex slices incubated with pyruvate formed glucose and lactate in relatively large and approximately equimolar quantities. The formation of these products involves two exclusively cytoplasmic NADH2-requiring reductions, catalysed by lactate dehydrogenase and triose phosphate dehydrogenase. From the rates of glucose and lactate formation it can be calculated that over 1000 (i-moles of NADH2 must have been produced in the cytoplasm/g. dry wt. of tissue/hr. When lactate is a gluconeogenic precursor the required NADH2 is generated in the cytoplasm, but, when a substrate more highly oxidized than glucose, such as pyruvate, is the precursor, there is no direct cytoplasmic source of NADH2. Quantitative data on the fate of pyruvate are in accord with the conclustion that the NADH2 was primarily formed intramitochondrially by the dehydrogenases of cell respiration, with pyruvate as the major substrate. Similar observations and conclusions apply to experiments with mouse-liver slices incubated with pyruvate, serine or aspartate. Addition of ethanol, which increases the formation of NADH2 in the cytoplasm, increased the formation from pyruvate of lactate but not of glucose. In view of the low permeability of mitochondria for NAD and NADH2 it must be postulated that special carrier mechanisms transfer the reducing equivalents of intramitochondrially generated NADH2 to the cytoplasm. Reasons are given in support of the assumption that the malate-oxalo-acetate system acts as the carrier. Various aspects of the generation of reducing power and its transfer from mitochondria to cytoplasm are discussed.
