Acetyl-CoA carboxylase from liver exhibits a linear inverse relationship between the ratio of enzymic activities at 0 and 2 mM
citrate and the extent of phosphorylation by its
kinase, and this
citrate activity ratio method was used to examine the effect of nutritional conditions on the phosphorylation state of the
enzyme. This method showed that the calculated phosphorylation state, being the extent of phosphorylation at sites accessible to carboxylase
kinase, was highest in the livers of starved rats, lower in those fed normally, and lower still in starved rats which had been refed for 48 h on a
fat-free diet. The actual values were 0.44, 0.26, and 0 mol of P/subunit, respectively, provided that liver samples were frozen rapidly to liquid
nitrogen temperatures and extracted with stopping
buffers at temperatures well below freezing. Normal homogenization with stopping
buffers (containing inhibitors for
protein kinases and
phosphatases) resulted in much higher calculated phosphorylation states. The effect of nutritional conditions on the phosphorylation state as estimated reported above was confirmed by purifying the carboxylase from livers of rats, measuring the amount of
phosphate which could be incorporated by carboxylase
kinase, and comparing this with the phosphorylation state calculated from the
citrate activity ratio method or the specific activity. Furthermore, treatment with
protein phosphatase of carboxylase from starved rats resulted in the largest increase in specific activity, that from the starved/refed rats in the least. Finally, the effects of
hyperglycemia on carboxylase and
phosphorylase characteristics in the livers of intact rats were ascertained by taking liver samples and preparing
crude extracts by the rapid freezing method described above.
Hyperglycemia caused a rapid increase in the activity of the carboxylase and a rapid decrease in its putative phosphorylation state as measured by the
citrate activity ratio method.
Phosphorylase was also dephosphorylated, as indicated by a decrease in
phosphorylase a activity. We conclude that the
citrate activity ratio method is a valid test for the phosphorylation state of
acetyl-CoA carboxylase in
crude extracts of tissue.