Localization of the reaction-centre subunits in the intracytoplasmic membranes of Rhodopseudomonas sphaeroides and Rhodopseudomonas capsulata [proceedings].

viously reported (Paine et al., 1979b), in the loss of 60% of their total nucleotide content, which is due to a loss of NAD+ and NADH (referred to as NAD throughout) (Table 1). Culture of hepatocytes for 24 h in media containing hyperphysiological concentrations of known precursors of NAD (Ijichi et al., 1966; Blake et al., 1967) shows that only culture medium containing nicotinamide is able to prevent the loss of NAD (Table 1). The failure of quinolinic acid to maintain NAD in cultured hepatocytes may be due to its lack of permeation into liver cells (Ijichi et al., 1966). This mechanism, however, would not seem to account for the failure of nicotinic acid to increase NAD, as hepatocytes cultured in medium containing [~arboxyl-~~Clnicotinic acid (sp. activity 53 pCi/mmol) accumulated the radiolabel proportionally to time over the first 4 h of culture and in a concentration-dependent manner. After 4 h of culture in medium containing 10 mM-niCOtiNC acid, hepatocytes contained I4C equivalent to 12nmol of nicotinic acid/mg of cell protein. The radioactivity associated with hepatocytes at this time remains constant for the next 20 h, suggesting that, if I4C is accumulated as nicotinic acid, the hepatocytes cultured in medium containing l0mM-nicotinic acid contain five times the amount of nicotinic acid required to replenish the NAD lost. The findings in Table 1 that only nicotinamide will increase the concentration of NAD in cultured hepatocytes are compatible with findings in experimental animals and with liver slices (Kaplan et al., 1956; Threlfall, 1959). Accordingly the present work shows that, in order to maintain NAD in cultured hepatocytes at the same concentration as that found in intact liver, it is better to incorporate nicotinamide than other precursors of NAD, such as nicotinic acid or quinolinic acid, into a cellculture medium.