ANAEROBIC GLYCOLYSIS AND FERMENTATIONS 63 



times and ranges of pH were used, so that the data are not completely com- 

 parable. Nevertheless, inspection of this table shows that the EM pathway 

 is quite sensitive to iodoacetate, and that in general it may be inhibited 

 over 90% by concentrations of 0.1 inM or above, even in cellular prepara- 

 tions where penetration of the iodoacetate may be limited. Concentrations 

 as low as 0.01 mM appear to inhibit around 40-50% and, from the work of 

 Meyer (1932) on lactic bacteria, possibly concentrations down to around 

 0.0005 mM may have some effect. 



What is an optimal concentration range of iodoacetate to use in blocking 

 the EjM pathway selectively ? Without considering other metabolic systems, 

 we can ask how low one can go and still expect marked block. These are 

 dangerous questions to try to answer because the reader may accept what- 

 ever is said with too much faith. But practically one wishes to have some 

 idea of what concentration to use in working with a new organism or tissue, 

 and particularly to avoid higher concentrations than necessary. There are 

 many factors which must be taken into account. (1) Are the conditions 

 aerobic or anaerobic, since we shall see that different sensitivities may be 

 exhibited? (2) Is the preparation cellular or acellular, i.e., are penetration 

 and external pH factors? (3) How long an incubation period with the in- 

 hibitor can be used in the particular experiment? (4) What is the inherent 

 sensitivity of the 3-PGDH for the preparation? (5) Are there protective 

 substances which may either react with iodoacetate or shield the active 

 center of 3-PGDH? (6) How much inhibition of the EM pathway is re- 

 quired for the particular purpose of the investigation? If one assumes an 

 extract or homogenate where penetration is not a factor, a concentration 

 range for iodoacetate of 0.05-0.2 mM might be suggested. In most cases 

 this will block over 90% and higher concentrations will be unnecessary; 

 we shall see that when the concentration approaches 1 mM or above it is 

 difficult to achieve a selective block of the EM pathway. It is much more 

 difficult to give an optimal range for cellular preparations, unless the cell 

 is permeable to the iodoacetate anion or the pH is favorable for penetra- 

 tion of the undissociated acid. In the latter case, a range similar to that 

 given above would be applicable, but when the intracellular concentration 

 is less than that in the medium, the concentration to be used in the me- 

 dium may have to be much greater than 0.2 mM. However, as may be seen 

 in Table 1-11 and from the discussion of specificity in Volume I (pages 

 877-878), it is often possible to inhibit cellular glycolysis significantly, and 

 reasonably specifically, at concentrations around 0.2 mM. The most im- 

 portant thing is not to use iodoacetate at concentrations of 1 mM or above 

 for the inhibition of the EM pathway unless there is good evidence that 

 this is necessary, for one reason or another. When one wishes to correlate 

 a cell function with the operation of the EM pathway, the use of unneces- 

 sarily high concentrations of iodoacetate makes the interpretation very dif- 



