PHOSPHATASES 439 



Normal tobacco leaves contain around 0.5-1 //mole/g wet weight glyco- 

 late and this level can be increased as much as 10-fold by placing them in 

 solutions containing the a-hydroxysuLfonates (Zelitch, 1959). Since glyco- 

 late is formed photosyntheticaUy, marked accumulation occurs only in the 

 light. The concentration of a-hydroxy-2-pyridinemethanesulfonate for max- 

 imal inhibition is near 10 m.M, indicating that penetration into the cells 

 is quite poor. At higher concentrations of inhibitor the glycolate level falls, 

 due presumably to inhibition of glycolate formation; in fact, even at 10 mM 

 there must be some inhibition since photosynthetic incorporation of C^Og 

 is inhibited about 33%. The pattern of C^* distribution is, however, more 

 markedly altered; in controls, glycolate-C^^ accounts for 5.5% of the labeling 

 but in inhibited leaves it is almost 50%. This is a good example of a spe- 

 cific analog inhibitor useful in studying the importance of an intermediate 

 in a complex metabolic pathway, and valuable information may result 

 from more detailed studies on photosynthesis. 



PHOSPHATASES 



These enzjines are commonly inhibited by the products of the hydroly- 

 sis. We have already noted the inhibitions of phospho-L-histidinol phos- 

 phatase, O-phosphoserine phosphatase (page 270), and glucose-6-phos- 

 phatase (page 442) by dephosphorylated products, and there are other 

 examples related to analog inhibition. Orthophosphate also frequently in- 

 hibits: The following may be cited as instances of well-marked inhibition 

 of different types of enzyme — calf intestinal phosphatase (Schmidt and 

 Thannhauser, 1943), mouse liver acid phosphatase (Macdonald, 1961), 

 mouse liver pyrophosphatase (Rafter, 1958), calf brain carbamyl and acyl 

 phosphatases (Grisolia et al., 1958), sweet potato phosphatase (Ito et al., 

 1955), and E. coli alkaline phosphatase (Garen and Levinthal, 1960). An- 

 other class of inhibitor comprises the phosphates that are substrates. Thus 

 sweet potato phosphatase with phenyl phosphate as substrate is competi- 

 tively inhibited by /^-glycerophosphate {Kf = 2 mM), pyrophosphate (K, = 

 = 0.33 mM), metaphosphate (^, = 3.2 mM), and ATP {K^ = 0.67 mM), 

 all of which are also substrates (Ito et al., 1955). Likewise the E. coli 

 phosphatase with p-nitrophenyl phosphate as substrate is competitively 

 inhibited by uridine phosphate {K^ = 0.044 raM), guanosine phosphate 

 {K^ = 0.046 mM), /^-glycerophosphate {K^ = 0.05 vnM), glucose- 1 -phos- 

 phate {K^ = 0.063 mM), and adenosine-5'-phosphate (Z,; = 0.093 mM) 

 (Garen and Levinthal, 1960). 



More interesting are the inhibitions by various anions that may be con- 

 sidered as analogs of either phosphate or the substrate phosphates. Thus 

 arsenate (Garen and Levinthal, 1960; Ito et al., 1955; Macdonald, 1961), 

 borate (Ito et al., 1955), and silicate (Umemura et al., 1961) inhibit various 



