In native rat liver microsomes glucose 6-phosphatase activity is depen
dent not only on the activity of the glucose-6-phosphatase enzyme (whi
ch is lumenal) but also on the transport of glucose-6-phosphate, phosp
hate and glucose through the respective translocases T1, T2 and T3. By
using enzymic assay techniques, palmitoyl-CoA or CoA was found to inh
ibit glucose-6-phosphatase activity in intact microsomes. The effect o
f CoA required ATP and fatty acids to form fatty acyl esters. Increasi
ng concentrations (2-50 mu M) of CoA (plus ATP and 20 mu M added palmi
tic acid) or of palmitoyl-CoA progressively decreased glucose-6-phosph
atase activity to 50% of the control value. The inhibition lowered the
V-max. without significantly changing the K-m. A non-hydrolysable ana
logue of palmitoyl-CoA also inhibited, demonstrating that binding of p
almitoyl-CoA rather than hydrolysis produces the inhibition. Light-sca
ttering measurements of osmotically induced changes in the size of rat
liver microsomal vesicles pre-equilibrated in a low-osmolality buffer
demonstrated that palmitoyl-CoA alone or CoA plus ATP and palmitic ac
id altered the microsomal permeability to glucose 6-phosphate, but not
to glucose or phosphate, indicating that T1 is the site of palmitoyl-
CoA binding and inhibition of glucose-6-phosphatase activity in native
microsomes. The type of inhibition found suggests that liver microsom
es may comprise vesicles heterogeneous with respect to glucose-6-phosp
hate translocase(s), i.e. sensitive or insensitive to fatty acid ester
inhibition.