6o8 V. T. NACHMIAS AND J. M. MARSHALL, JR. 



EFFECTS OF PH ON BINDING 



The effects of pH on the binding of ferritin and of methyl ferritin to 

 the cell surface were studied. Because the proteins had an intense orange 

 colour, the results of binding and washing experiments could be followed 

 by direct observation of the living cells. The results were confirmed by 

 electron microscopy. 



At pH 4, when both proteins were positively charged, both were bound, 

 and both invoked the pinocytosis response. At neutral or slightly acid pH, 



Fig. 2. Binding of ferritin to surfacL- coat ot amoeba C/kius cJiaos, at pH 4. The 

 binding persists after washing at pH 4 for 5 to 10 min. 



when its particles carried a net negative charge, ferritin was not bound 

 and did not invoke the pinocytosis response. 



Methyl ferritin was too insoluble at neutral pH to permit a direct 

 comparison of its binding with that of ferritin, but a clear difference in the 

 reversibility of binding was seen in washing experiments. Once bound to 

 the cell surface at pH 4, neither protein was removed by washing in the 

 cold at pH 4. When the washing was done above pH 5, ferritin was quickly 

 removed, but methyl ferritin was not. 



Figure 2 is an electron micrograph which shows ferritin bound to the 

 cell at pH 4, and remaining bound after washing at the same pH. Note that 



