Pyridine nucleotide levels as a function of growth in normal and transformed 3T3 cells.

The levels of NAD (NAD⁺ + NADH) and NADP (NADP⁺ + NADPH) and their redox states were measured as a function of growth in 3T3 mouse fibroblasts which exhibit density-dependent inhibition of growth and SV40 (simian virus #40)-transformed 3T3 cells (SVT2) which have lost this property. The levels were related to cell numbers, protein content, and rates of DNA synthesis. At corresponding cell densities, 3T3 cells contain approximately twice as much total protein as SVT2 cells. The levels of NAD relative to total cellular protein are density dependent in both 3T3 and SVT2, increasing with increasing cell density. Over a 30-fold range of cell densities, the NAD levels in 3T3 increase 2.4-fold, while the levels in SVT2 increase 1.6-fold. The levels of NAD are very similar in dividing 3T3 and SVT2 cells at corresponding cell densities; however, a marked increase in the levels of NAD is observed in 3T3 cells, but not in SVT2 cells, at cell densities just prior to where 3T3 cells enter density-dependent inhibition of growth. This increase in NAD levels is correlated with the cessation of DNA synthesis. The NAD pools are 15–25% NADH for 3T3 and 5–15% NADH for SVT2. NADP levels relative to protein in 3T3 and SVT2 are less density dependent, with overall increases of 1.3- and 1.2-fold, respectively, observed over the range of cell densities examined. NADP levels relative to protein are nearly twice as high in SVT2 cells as in 3T3 cells of corresponding cell densities. The NADP pools are approximately 70–80% NADPH in both cell types.