Adenosine 5'-monophosphate-removing system in fructose-1,6-bisphosphatase assay mixture: a new approach

Metformin reduces liver glucose production by inhibition of fructose-1-6-bisphosphatase

Metformin is a first-line drug for the treatment of individuals with type 2 diabetes, yet its precise mechanism of action remains unclear. Metformin exerts its antihyperglycemic action primarily through lowering hepatic glucose production (HGP). This suppression is thought to be mediated through inhibition of mitochondrial respiratory complex I, and thus elevation of 5'-adenosine monophosphate (AMP) levels and the activation of AMP-activated protein kinase (AMPK), though this proposition has been challenged given results in mice lacking hepatic AMPK. Here we report that the AMP-inhibited enzyme fructose-1,6-bisphosphatase-1 (FBP1), a rate-controlling enzyme in gluconeogenesis, functions as a major contributor to the therapeutic action of metformin. We identified a point mutation in FBP1 that renders it insensitive to AMP while sparing regulation by fructose-2,6-bisphosphate (F-2,6-P2), and knock-in (KI) of this mutant in mice significantly reduces their response to metformin treatment. We observe this during a metformin tolerance test and in a metformin-euglycemic clamp that we have developed. The antihyperglycemic effect of metformin in high-fat diet-fed diabetic FBP1-KI mice was also significantly blunted compared to wild-type controls. Collectively, we show a new mechanism of action for metformin and provide further evidence that molecular targeting of FBP1 can have antihyperglycemic effects.

Ostrich fructose 1,6-bisphosphatase: distribution of activity and purification of the liver isoenzyme

1. Fructose 1,6-bisphosphatase was assayed in crude extracts of physiologically important organs and tissues in the ostrich. 2. Highest activity was found in liver and lowest in brain tissue. 3. No activity was detected in the heart, gizzard or adrenals. 4. The enzyme was purified in homogeneous, apparently undegraded form from liver utilizing Blue dextran-Sepharose affinity chromatography. 5. The enzyme is similar to mammalian fructose 1,6-bisphosphatase in many respects including its indispensability of Mg2+ for catalytic activity. 6. Relative molecular weight of the native enzyme and its subunit is about 150,000 and 35,000 respectively. 7. The amino acid composition of ostrich liver fructose 1,6-bisphosphatase is distinctly different from that of the chicken muscle enzyme, but compares favourably with the composition of the rabbit liver enzyme. 8. The purified enzyme is devoid of tryptophan.

Sensitivity of fructose-1,6-biphosphatase from yeast, liver and skeletal muscle to fructose-2,6-biphosphate and 5'-adenosine monophosphate

As a prerequisite for future studies on the possible effect of sulphite, an anti-microbial agent, on gluconeogenesis in yeast, a comparative study of fructose-1,6-bisphosphatase (FBPase), a key enzyme of gluconeogenesis, from yeast, liver and skeletal muscle is reported. In contrast to FBPase from yeast or liver, FBPase from skeletal muscle is approximately 1000-fold more sensitive to inhibition by 5' adenosine monophosphate and 30 to 250-fold less sensitive to inhibition by fructose-2,6-bisphosphate. The kinetic properties of the FBPases, determined by the ratios R(Mg2+/Mn2+) and R (pH 7/9) of the enzyme activities, measured at 10 mM Mg2+ and 2 mM Mn2+ and at pH 7.0 and 9.0, respectively, show a drastic difference between the skeletal muscle and the yeast or liver enzymes. The data support the idea that the enzymes from yeast and liver function in gluconeogenesis, whereas the enzyme from skeletal muscle is involved in other biological functions.

Fructose 1,6-bisphosphatase in rat liver cytosol: interactions between the effects of K+, Zn2+, Mn2+, and fructose 2,6-bisphosphate as measured in a steady-state assay

Fructose 1,6-bisphosphatase activity was determined in rat liver cytosols using glyceraldehyde 3-phosphate as primary substrate. Fructose 1,6-bisphosphate was formed in situ and steady-state concentrations ranging from 1 to 30 microM were observed depending on the activity of fructose 1,6-bisphosphatase and the concentration of added glyceraldehyde 3-phosphate. The system was free of contaminating low-molecular-weight compounds, divalent cations, and chelators. Under these conditions, fructose 1,6-bisphosphatase was inhibited by K+ (less than or equal to 200 mM). This inhibition was due to a reduction of V and was observed in presence of low (0.4 mM) and high (5 mM) concentrations of Mg2+. In presence of 0.4 mM Mg2+, 1 microM Zn2+ inhibited fructose 1,6-bisphosphatase by 50%; the same effect was obtained with 0.3 microM Zn2+ when the system was supplemented with 100 mM KCl. On the other hand, 0.2 microM Zn2+ enhanced the inhibitory effect of K+ and decreased the concentration of K+ yielding half-maximal inhibition from 175 to 100 mM when measured at 0.4 mM Mg2+. The effect of Zn2+ on the inhibition by K+ could be abolished by Mn2+ (less than 5 microM) or by 5 mM Mg2+. One hundred millimolar K+ enhanced the inhibition of fructose 1,6-bisphosphatase by fructose 2,6-bisphosphate and changed the type of inhibition from mainly competitive to a mixed-type inhibition (increase of Km, decrease of V). Mn2+ (less than 10 microM) reduced the effect of fructose 2,6-bisphosphate, especially in the presence of K+. It is proposed that K+ and Mn2+ may play a role in the regulation of gluconeogenesis.