The effect of glucose on the activity of phosphofructokinase in the mucosa of rat small intestine

Abstract

In common with other phosphofructokinase isoenzymes, phosphofructokinase in the epithelial cells of rat small-intestinal mucosa is activated by fructose 2,6-bisphosphate. However, fructose 2,6-bisphosphate was found not to be present in mucosa as judged by three criteria: (1) chromatography on Sephadex G-25 of crude mucosal extracts from fed rats did not result in a decrease, or indeed any change, in the activity of phosphofructokinase under suboptimal conditions at pH7; (2) ultrafiltrates of mucosal extracts did not possess any acid-labile activating activity when tested against chromatographed liver phosphofructokinase; (3) phosphofructokinase-2 activity was not detectable in mucosal extracts. Furthermore, the perfusion in vitro of isolated loops of jejunum or the incubation of mucosal scrapings from either fed rats or rats starved for 48 h showed that the activity of mucosal phosphofructokinase is not subject to short-term regulation by glucose. These observations are consistent with the view that phosphofructokinase is the rate-limiting enzyme of glycolysis in intestinal mucosa and account for the fact that the rate of glucose utilization by rat small intestine is not very responsive to changes in the concentration of glucose in the lumen.

Fructose 2,6-bisphosphate in rat small intestine

“…Subsequently, Louis et al (1985) reported that fructose 2,6-bisphosphate was present in isolated enterocytes at concentrations comparable to those observed for liver (Hue, 1982;Pilkis et al, 1983). In this letter we report that fructose 2,6-bisphosphate is in fact present in mucosa under the conditions used by Jamal et al (1984), but that its concentration is one order of magnitude less than under those used by Louis et al (1985). The difference reflects differences in intestinal preparation and also in the composition of the commercial diets with which the rats were fed.…”

“…This value is 2.4-fold less than for whole intestine and 9.7-fold less than for enterocytes from rats fed solely on the Bantin & Kingman diet. We therefore conclude that the concentration of fructose 2,6-bisphosphate in intestine under the conditions used by Jamal et al (1984) was an order of magnitude less than under the conditions used by Louis et al (1985). Different types of intestinal preparation, when used in conjunction with different brands of commercial chow diet, can result in very different rates of glucose metabolism and fructose 2,6-bisphosphate concentration.…”

“…Fructose 2,6-bisphosphate in rat small intestine In a previous report from this laboratory, Jamal et al (1984) attempted to establish a role for fructose 2,6-bisphosphate in the regulation of phosphofructokinase from intestinal epithelial cells. However, they were unable to detect any fructose 2,6-bisphosphate in mucosa taken from luminally-perfused whole intestine, although they were able to detect it readily in liver, as expected.…”

“…They found, however, that the activity ratios were unaffected by chromatography of extracts on Sephadex G-25 and therefore that the difference in regulatory properties was unlikely to be caused by a lowMr effector such as fructose 2,6-bisphosphate. Jamal et al (1984) used mucosa taken from luminallyperfused whole intestine rather than isolated enterocytes for their experiments. If it were to be the case that mucosa from whole intestine contained the same concentration of fructose 2,6-bisphosphate (13.5 nmol/g dry wt.)…”

Fructose 2,6-bisphosphate in isolated rat enterocytes

“…for at least six cell preparations. fructose 2,6-bisphosphate nor 6-phosphofructo-2-kinase were detected in intestinal mucosa (Jamal et al, 1984). However, mucosal phosphofructokinase, like that of other tissues (Hers et al, 1982), was stimulated by fructose 2,6-bisphosphate (Jamal et al, 1984).…”