4 MAIRICK .1. HKSSMAN 



at the diphosphate level and show that optimal synthesis requires the 

 participation of ATP, IMg**, and reduced lipoic acid. These results are 

 presented in Table I. It is interesting to note that reduced lipoic acid is 

 an essential component of the system and cannot be replaced by TPNH, 

 DPNH, glutathione, ascorbic acid, 2,3-mercaptopropanol, or oxidized 

 lipoic acid. The authors suggest that the cfTect of lipoic acid may be 



TABLE I 



KEyl IKEMENTS FOR dCDP FORMATION WITH A 



PiRiFiED Fraction from Escherichia coli "•'' 



" From Bertani el al. (1961). 



'' The complete reaction mixture contained 0.04 /imole of CDPH^ (3 X 10" cpm per 

 Mmole), 0.5 ^mole of ATP, 1.5 /umoles of MgClo, 0.2 yumoles of reduced DL-lipoic acid, 2.5 

 Aimoles of tris-HCl buffer, pH 7.85, 0.1 /umole of mercaptoethanol, and 0.11 mg of frac- 

 tion B in a final volume of 0.1 ml. 



indirect, mediated through a flavin coenzyme, but they could not rei)lace 

 lipoic acid by FAD or FADHo. The requirement for ATP suggests the 

 participation of an "activated" intermediate during the reductive process, 

 possibly a phosphorylated, pyrophosphoiylated, or adenylated derivative 

 at the 2'-position (Bertani et al., 1961). This same enzyme fraction 

 catalyzes the reduction of UDP to deoxy-UDP, again at the diphosphate 

 level (Table II). 



The specificity of this enzyme in respect to nuclco.^ide diphosphates 

 has not been described and the question may be asked whether the same 

 enzyme or similar enzymes account for the net formation of purine 

 deoxyribonucleotides. Indications that the purine deoxyribosyl com- 

 pounds are fonued directly from ribosyl derivatives have been reported 

 from experiments in whole cells (Roll cf a/.. 1956; McNutt, 1958b) and 

 in cell extracts (Reichard, 1960). This probably represents the major 

 pathway for the fonnation of the purine deoxyribonucleotides, but the 

 possibility of a contribution by the transdeoxyribosidase of McNutt 



