Ionic Homeostasis in White Blood Cells 
NADPH oxidase. We are in the process of explor- 
ing the relationship between these events, with 
particular interest in whether the putative chan- 
nels are a component of the oxidase complex or 
whether assembly of the latter is required for ac- 
tivation of the conductance. 
For this purpose, we have initiated studies us- 
ing-cells from patients with chronic granuloma- 
tous disease. These cells are defective in specific 
components of the NADPH oxidase. In future ex- 
periments we will attempt to detect the conduc- 
tance electrophysiologically and to reconstitute 
its activity in cell-free systems. If a linkage be- 
tween the two processes is established, purified 
or recombinant components of the oxidase will 
be used. 
Proton Pumps in Cytoplasmic 
and Organellar pH Regulation 
Recent experiments have also suggested that a 
third mechanism of proton extrusion is opera- 
tional in stimulated leukocytes. Pharmacological 
evidence indicates that this latter pathway may be 
a proton-pumping ATPase of the vacuolar type. As 
described for the other systems, the activity of the 
pumps becomes clearly apparent after stimula- 
tion. At present, neither the subcellular location 
of the pumps is known nor the mechanism of ac- 
tivation understood. We will try to determine 
whether activation results from post-translational 
modification of pumps present in the relevant 
membrane or whether translocation between in- 
active and active compartments occurs. Immuno- 
chemical and molecular biological means will be 
used to localize the pumps and identify the 
type(s) of isozyme involved. 
Proton pumps are also seemingly responsible 
for phagosomal acidification. We are currently in- 
terested in the source of the pumps that underlie 
this process and their mode of activation. We are 
also planning to study the determinants of the 
internal pH of the phagosome and other endo- 
membrane compartments. Our current evidence 
suggests that differential counterion permeabil- 
ity, which was claimed to be the main source of 
pH heterogeneity, is not an important factor dic- 
tating intraorganellar pH. Differential pH sensitiv- 
ity of the pumps, due to varying subunit compo- 
sition, or a regulated proton leak permeability are 
our preferred hypotheses. 
These studies are expected to contribute to our 
understanding of immune cell function and intra- 
cellular pH regulation in these and other cells. 
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