PROTEIN UPTAKE IN AMOEBAE 607 



of micelles of ferric hydroxide and ferric phosphate. The particles are of 

 uniform size and shape, and the internal cluster of dense iron micelles 

 makes it possible to identify in high resolution electron micrographs even 

 a single particle [ii, 12]. The protein coat is responsible for the electro- 

 chemical properties of ferritin, such as electrophoretic mobility and 

 solubility. Ferritin behaves, therefore, as a typical protein ampholyte, with 

 an isoelectric point of 4-4 [13]. 



A methylated deriyative of ferritin was prepared by esterifying the 

 protein in acid methanol [14]. The effect of methylation was judged by 

 comparing at several pH \alues the solubility of the product with that of 

 the original protein. Solubility (in optical density units) was measured by 

 determining the absorbance at 280 m/i of supernatant solutions in 

 equilibrium with precipitated proteins (Fig. i). 



pH 



Fig. I. Solubility of ferritin (curve A) and of methylated ferritin (cur\e B), as 

 a function of pH. Ordinate: solubility in optical density units, at 280 m/t. 



No direct measure of the percentage of carboxyl groups blocked was 

 attempted, but the solubility curves demonstrated that the charge proper- 

 ties were significantly modified by methylation. The solubility of normal 

 ferritin above pH 5 is almost entirely due to the ionization of carboxyl 

 groups. When these are blocked by extensive methylation, the solubility at 

 pH 6 to 7 drops to a very low value (about io~^",) by weight). In electron 

 micrographs, the unit particles of methyl ferritin appeared the same as 

 those of ferritin. It seemed safe to conclude that methylation did not 

 grossly alter the structure of the ferritin particle, and that any difference 

 in binding or uptake bv amoebae could be correlated with the specific 

 charge effect of blocking carboxyl groups. 



