Liver and muscle isozymes have been found in vertebrate
Liver and muscle isozymes have been found in vertebrate tissues , , . The liver FBPase is recognized as a regulatory enzyme of gluconeogenesis. The muscle isozyme participates in the glycogen synthesis from lactate and in the regulation of glycolysis , , . The basic difference between the liver and muscle isozymes concerns their sensitivity to AMP inhibition. In the case of the muscle enzyme, the I0.5 value for AMP is about 0.1 μM, and it is 10–100 times lower than the same value determined for the liver isozyme , , , .
Tissue distribution of FBPase isozymes has been investigated. The liver isozyme has been found primarily in gluconeogenic tissue like liver, kidney, and lung. In skeletal muscle tissue only the muscle isozyme is expressed, but in other tissues simultaneous expression of the two isozymes has been observed .
Not many papers are available on cellular and subcellular FBPase isozymes localization. Schmoll et al.  have found FBPase to be an astrocyte-specific enzyme. Gizak et al.  have located FBPase in pneumocyte II. Saez et al.  have reported that in hepatic and renal K02288 mg FBPase is located in the perinuclear area. Recently Gizak et al. have located FBPase on both sides of the Z-line of skeletal muscle . The primary aim of the present paper is to locate FBPase in subcellular structures of cardiomyocytes. Evidence for localization of FBPase in the nuclei of cardiomyocytes is presented, and physiological meaning of this finding is discussed.
Materials and methods
Discussion Presence of FBPase in the nuclei of cardiomyocytes was detected by light and electron microscopy and confirmed by Western blot. Determining the I0.5 for FBPase for AMP from the heart homogenate and from the purified nuclear fraction (which was the same as for the purified enzyme, in the range of experimental error) proved FBPase’s presence. Although FBPase activity in nuclei consists of ca. 1% of the total enzyme activity, its concentration therein is four to five times higher than in the cytosol. Thus, non-covalent binding of FBPase with nuclear structures may be expected. Detection of FBPase activity only in nuclei pre-treated with Triton supports the above hypothesis. The presence of FBPase in the nuclei of cardiomyocytes raises a question concerning its physiological role. Several enzymes of carbohydrate metabolism have so far been found in cell nuclei: glucokinase , aldolase , glyceraldehyde-3-phosphate dehydrogenase (GAPDH), lactate dehydrogenase (LDH), phosphoglycerate kinase (PGK) (for review see: ), and glycogen synthase , and their physiological roles have been discussed. Unlike glycogen synthase, whose function is supposedly the same in the cytosol and the nucleus, the other enzymes’ physiological roles seem to vary from cytosol to the nucleus. It has been hypothesized that PGK may participate in DNA synthesis and cell cycle progression . GAPDH recognizes the sequence and structural features of the RNA and is involved in transcription ; LDH is a recognized stabilizing nuclear factor, and it participates in DNA reparation . One of the signals directing protein to the nucleus is phosphorylation. There is no evidence on in vivo phosphorylation of liver FBPase. On the other hand, Rakus et al.  have found that rabbit muscle isozyme is phosphorylated. Phosphorylated FBPase has higher affinity to aldolase, which increases a muscle cell’s gluconeogenic capacity . In the liver, stimulation of gluconeogenesis goes via phosphorylation of fructose-2,6-bisphosphatase/6-phosphate-2-fructokinase (FBPase2/PFK2). The muscle isozyme of FBPase2/PFK2 cannot be phosphorylated, therefore, different regulation of gluconeogenesis may be expected. The activation of muscle gluconeogenesis may proceed via phosphorylation of FBPase. That might be the signal inducing the movement of the enzyme to the nucleus where it could induce the expression of its own gene, thus increasing the cell’s gluconeogenic capacity, although a possibility that FBPase influences the expression of other genes cannot be ruled out.