144 



MICROSOMAL PARTICLES 



sulfate, and sulfide, the initial uptake is unaltered. This finding was reported 

 previously by Billen and his associates [1, 2, 3, 4], and we confirm it. 



For the sulfate and sulfide data a simple explanation can be advanced. It 

 is best seen diagrammatically as in figure 7. We suppose that the synthesis of 

 protein is determined by the presence of intact units of the size stated in the 

 previous paper for methionine. However, the protein made can be essential 

 for the enzymatic reduction of sulfate or combination of reduced sulfur with 

 other elements to form intermediates, finally ending in methionine, cysteine, or 

 glutathione. Each enzyme is a protein and probably requires methionine or 

 cysteine, i.e., sulfur in the right form. The entire uptake of sulfur is thus sensi- 

 tive in three ways: first as final incorporation into protein, second as requiring 

 the availability of a suitable enzyme for a needed process, third in terms of the 

 synthesis of such enzymes. The sensitivity of methionine incorporation then 

 requires only the functioning of one ribosome. Sulfate will not be incorporated, 

 however, if enzymes are inactivated (experience shows this inactivation to be 

 relatively difficult, as each enzyme molecule is small), or if enzyme synthesis 

 is stopped, or if protein synthesis is stopped. Thus a much larger inactivation 

 volume for sulfate uptake can be predicted. From the ratio of the inactivation 

 volume for sulfate uptake, after the initial stage is over, to that for methionine 

 uptake, the number of steps involved can be estimated. From table 1 the steps 

 from sulfate to methionine are 8.7, and from sulfide to methionine 5. The 

 process is shown schematically in figure 7. We make no claim for correct inter- 

 mediary biochemistry. If the explanation we propose is right it could be checked 

 against known intermediary processes by means of competition techniques. 

 This check is planned for future work. 



One question naturally asked concerns any change in the fractions in the 

 cell after irradiation. We studied the four fractions: alcohol-soluble, cold-TCA- 

 soluble, hot-TCA-soluble, and residue. The results are shown in table 2. We 

 found the relative proportion of S 35 in the hot-TCA-soluble and residue frac- 

 tions to depend somewhat on the time and temperature of exposure to hot TCA, 

 and we do not regard the variations in the hot-TCA fraction as significant. 

 Possibly the fall in the alcohol-soluble fraction after heavy irradiation in sulfate 



