Glucose-6-phosphatase distribution

Leskes, A., Siekevitz, P., Palade, G.E. Differentiation of endoplasmic reticulum in hepatocytes. I. Glucose-6-phosphatase distribution in situ. J. Cell Biol. 49, 264-287 (1971)... 

Phosphoenolpyruvate carboxykinase (PEPCK) deficiency is distinctly rare and even more devastating clinically than deficiencies of glucose-6-phosphatase or fructose-1,6-bisphosphatase. PEPCK activity is almost equally distributed between a cytosolic form and a mitochondrial form. These two forms have similar molecular weights but differ by their kinetic and immunochemical properties. The cytosolic activity is responsive to fasting and various hormonal stimuli. Hypoglycemia is severe and intractable in the absence of PEPCK [12]. A young child with cytosolic PEPCK deficiency had severe cerebral atrophy, optic atrophy and fatty infiltration of liver and kidney. 

Because the metabolism of DEHP was catalyzed by so many fractions of the trout liver homogenate, these fractions were characterized by measurement of marker enzymes to determine which organelles actually were responsible for the observed DEHP metabolism. Succinic dehydrogenase activity was used as a marker for mitochondria, whereas glucose-6-phosphatase was used as a marker for microsomes. The distribution of DEHP oxidase activity (production of polar metabolites 1 and 2 with added NADPH) and of DEHP esterase activity (production of monoester without added NADPH) were also determined. It was found (Figure 2) that the distribution of DEHP oxidase activity parallels the distribution of microsomal activity and the distribution of DEHP esterase activity parallels the distribution of microsomal activity, but is also present in the cytosol fraction. 

Subcellular Distribution of Inorganic Pyrophosphatase, PPi-GLUcosE Phosphotransferase, and Glucose-6-P Phosphohydrolase Activities of Rat Liver Microsomal Glucose-6-Phosphatase ... 

Proximal tubule cells in culture should have retained functional attributes such as (1) polar architecture and junctional assembly of epithelia and correct membrane distribution of enzymes and transport systems (2) vectorial transport of solutes and water, manifested by the formation of domes when cultured on solid supports [81] and the generation of transepithelial electrophysiological properties [82, 83] due to the expression of proximal tubule specific claudins 2- and 10 [84, 85] (3) cellular uptake of xenobiotics from either the apical or basolateral side, as observed in vivo and (4) expression of nephron segment-specific characteristics, i.e., distinct expression of differentiation markers, metabolic and transport properties, and hormone responsiveness. Such markers include the expression of the brush border enzymes alkaline phosphatase, leucine aminopeptidase, and y-glutamyl transferase [4, 86], In addition, proximal tubule cells should possess Na+,K+-ATPase activities, Na+-dependent glucose, and p-aminohippurate transport. Proximal tubule cells increase cAMP levels in response to parathyroid... 

In practice, using a normal chemistry panel and complete blood count it is not unusual to have 30 potential covariates, everything from sodium ion concentration to alkaline phosphatase activity. Early in PopPK analyses it was not unusual to screen every single covariate for their impact on the model. But a model might end up having a volume of distribution as a function of chloride ion concentration or clearance that is a function of glucose concentration. Physiologically, these covariates are nonsensical. Ideally at the end of model... 

The third bypass reaction requires the hydrolysis of glucose-6-P04 to glucose, which then can leave the liver and enter the blood for distribution to other tissues. This reaction requires a third new enzyme, glucose-6-phosphatase ... 

Studies of the distribution of glucose-6-phosphatase in hepatocytes after birth support the second view. During hepatocyte development in the fetus and the newborn rat, the rough endoplasmic reticulum appears first. The smooth endoplasmic reticulum expands a few days before birth, and glucose-6-phosphatase appears after birth. Electron microscopic localization of the glucose-6-phosphatase shows that the enzyme appears simultaneously in most hepatocytes in all parts of the endoplasmic reticulum. Therefore, there are no growing points for glucose-6-phosphatase, and the new enzyme molecules appear to be continuously inserted within the old framework. 

G. Weber, C. Allard, G. de Lamirande and A. Cantero, Liver Glucose-6-Phosphatase Activity and Intracellular Distribution after Cortisone Administration, Endocrinology 58, 40-50 (1956). 

Fig. 8. Electronmicrograph of a hepatocyte from an adult liver incubated for 30 minutes for glucose-6-phosphatase activity. The deposits of enzyme reaction are evenly distributed throughout the endoplasmic reticulum. N, nucleus nm, nuclear membrane rm, rough membranes of the endoplasmic reticulum sm, smooth membranes m, mitochondria [Courtesy of the Journal of Cell Biology (Leskes et al., 1971).]...

Fiq. 3. Distribution of DNA and different tracer activity in centrifugal field. SDHG, succinic dehydrogenase Ac Pase, acid phosphatase G-6-Paae, glucose-6-phosphatase. 

Fig. S. Distribution, of acid phosphatase and giucose-6-phosphataae activity in sub-fractions of homogenate frctn human liver shown in relation to the capacity of Uie subfractions for stimulating m.ierosoraal synthesis of tauro- and glyeocholie acids. C— , Synthesis-increasing capacity, m/jmoles conjugates/mg protein from the respective fractions added to basal quantity of microsome protein. ---- , Acid phosphatase activity. Glucose-S hosphataae activity. N, nuclear fraction ...

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