Intestine glucose-6-phosphatase

Enzyme labels are usually coupled to secondary antibodies or to (strept)avidin. The latter is used for detection of biotinylated primary or secondary antibodies in ABC methods (see Sect. 6.2.1). Enzyme labels routinely used in immunohisto-chemistry are horseradish peroxidase (HRP) and calf intestinal alkaline phosphatase (AP). Glucose oxidase from Aspergillus niger and E. coli (3-galactosidase are only rarely applied. 

Immunoenzymatic staining methods utilize enzyme substrate reactions to convert colorless chromogens into colored end products. Of the enzymes used in these applications, only horseradish peroxidase and calf intestine alkaline phosphatase will be considered in some detail. Because of its low sensitivity, glucose oxidase (Aspergillus niger) is only rarely used today. 

A systematic study with modern methods has been undertaken by Lincheer et al. (L14) of the relationship of intestinal alkaline phosphatase and glucose absorption. Apparently glucose is capable of stimulating the intestinal mucosa to secrete an L-phenylalanine-sensitive alkaline phosphatase. 

Because of the need for sensitivity, most labels for DNA analysis are enzymes or highly efficient fluor-ophores such as phycobiliproteins. The most commonly used enzyme labels are calf intestinal alkaline phosphatase and horseradish peroxidase (Table 2). Beta-galactosidase is used primarily for gene expression monitoring. Other enzymes such as glucose-6-phosphate dehydrogenase have not fovmd as wide utility as phosphatase and peroxidase. 

The hydrolysis of glucose-l-phosphate by acid, prostatic acid phosphatase, and intestinal alkaline phosphatase has been studied by Cohn who used H20 in the medium. Her fundamental work has shown that upon acid hydrolysis the C—0 bond in glucose-l-phosphate is ruptured, but on enzymatic hydrolysis it is the 0—P bond that is cleaved. In the first case no 0 was detected in the inorganic phosphate formed, but in the second case the 0 was taken up. 

Enzymatic markers used in immunohistochemistry Horseradish peroxidase (HRP) and calf intestinal or E.coli alkaline phosphatase (AP). Glucose oxidase from Aspergillus niger and E.coli /3-galactosidase are only rarely applied. 

Studies on the transferase action of milk and intestinal phosphatases have shown that compounds such as glucose, glycerol, and propanediol can accept a phosphoryl residue from a wide variety of donors (IBS). The overall reaction is therefore transfer of a phosphoryl group from a donor of type (II) where X is F, 0, S, or N and R is H or an alkyl substituent, etc., or may even be absent, to an acceptor of type R —OH... 

It appears to the author that the existence of a distinct glucose-6-phosphatase is well established for endoplasmic reticulum of liver, kidney, small intestine, and pancreas but additional studies appear to be required before a similar conclusion may be reached regarding the enzyme from other sources. It should be pointed out, however, that while glucose-6-P hydrolysis by enzymes from many sources may not result from specific glucose-6-phosphatase, this hydrolysis in these tissues may nonetheless be of metabolic significance. 

Comparison of Levels of Phosphotransferase and Phosphohydrolase Activities of Glucose-6-Phosphatase in Liver, Kidney, and Small Intestine of Rabbit"... 

Reported Presence of Glucose-O-phosphatase in Vertebrate Small Intestine... 

I von Gierke la glucose-6-phosphatase Ib translocase for glucose 6-phosphatase Ic phosphate/pyrophosphate translocase Liver, Iddney, intestines... 

Glucose-6-phosphatase is part of a multi-component integral membrane protein system (Figure 15-3) located in the endoplasmic reticulum of liver, kidney, and intestine but not of musele or adipose tissue. 

The answer is d. (Murray, pp 505-626. Scriver, pp 4029-4240. Sack, pp 121-138. Wilson, pp 287-320.) Although the liver is the major site of the formation of free glucose to maintain blood glucose levels, the kidneys and intestinal epithelium (e.g., duodenum, jejunum, and ileum) may also release glucose. All of these tissues contain the enzyme glucose-6-phosphatase, an endoplasmic reticulum enzyme that dephosphorylates glucose and allows it to be transferred out of the cells. No other tissues in mammals contain this enzyme. 

---