GLUCOSE-6-PHOSPHATASE IN HUMAN JEJUNAL MUCOSA. LACK OF ACTIVITY IN GLYCOGENOSIS OF CORI'S TYPE 1

Endoplasmic reticulum phosphate transport

The major role of the liver endoplasmic reticulum phosphate/pyrophosphate transport proteins is the regulation of blood glucose levels. The glucose-6-phosphatase enzyme is an endoplasmic reticulum enzyme system which hydrolyzes glucose-6-phosphate to glucose and phosphate. Glucose-6-phosphatase is the terminal step of both gluconeogenesis and glycogenolysis. The glucose-6-phosphatase enzyme is a very hydrophobic membrane protein and its active site is inside the lumen of the endoplasmic reticulum. The substrates and products of the enzyme therefore have to cross the endoplasmic reticulum membrane. The glucose-6-phosphatase enzyme is associated with a calcium binding protein (SP). There are also transport proteins for the substrate glucose-6-phosphate (T1) and the products phosphate (T2) and glucose (T3). There appear to be at least two different liver endoplasmic reticulum proteins that can transport phosphate. One of the proteins T2b can also transport pyrophosphate and carbamyl phosphate which are also substrates for the glucose-6-phosphatase enzyme. The metabolic regulation, genetic deficiencies, ontogeny and tissue distribution of the endoplasmic reticulum T2 proteins will be described.

The glucose-6-phosphatase enzyme in developing human trachea and oesophagus

Glucose-6-phosphatase is an endoplasmic reticulum system which is found primarily in liver and kidney. Recently, it has become clear that it is also present in lower amounts in a variety of other tissues. Previous histochemical studies of glucose-6-phosphate hydrolysis in trachea have given equivocal results and only one study on adult oesophagus has shown glucose-6-phosphatase, enzymatic activity but without cellular localization. We have now shown, using microassay techniques, that microsomes isolated from human foetal trachea and oesophagus both contain low levels of specific glucose-6-phosphatase activity (mean = 0.9 and 1.5 nmol min-1 mg-1 microsomal protein, respectively) which are less than 10% of the levels in microsomes of human foetal liver of similar age. In the developing trachea, glucose-6-phosphatase immunoreactivity has been found, using a monospecific antibody to the catalytic subunit of the glucose-6-phosphatase enzyme, to be first present at 10-11 weeks' gestation, and thereafter in foetal life, predominantly present in ciliated cells, with smaller amounts in non-ciliated secretory cells, duct lining cells, and occasional basal cells. The foetal oesophageal epithelium is transiently ciliated from 10 to 11 weeks' gestation, but ciliated cells are gradually replaced by squamous cells from 14 to 16 weeks onwards. Glucose-6-phosphatase immunoreactivity in human foetal oesophagus is predominantly confined to ciliated cells, but non-ciliated luminal cells are also reactive, as are occasional basal cells. Mucus secretory cells in foetal trachea and oesophagus are immunonegative, as is the entire epithelium of both organs in the embryo (up to 56 postovulatory days.

Effect of glutaraldehyde and lead on the activity of hepatic glucose-6-phosphatase. A biochemical and cytochemical study

The sensitivity of mouse liver glucose-6-phosphatase activity towards glutaraldehyde fixation has been analysed by biochemical and cytochemical means. The degree of enzymatic inhibition and various enzymatic properties have been studied. Several differences have been observed in the Km determination, the sensitivity to pH 5 and the activity related to pH between fixed and unfixed enzymes. The role of Pb++ ions in the cytochemical media has also been estimated. It is concluded that several enzymatic differences appear between fixed and unfixed enzymes and that the inhibition by Pb ions is dependent on the buffer and on the amount of substrate used.

Barium enema findings in type I hepatorenal glycogen storage disease

Four children with Type I hepatorenal glycogen storage have been studied by barium enema. All showed strikingly similar changes of a smooth-walled, slightly narrow but normal length colon without any haustration. The findings simulated colitis but the patients had either mild diarrhea or no gastrointestinal complaints. Small bowel series was normal. Endoscopy and biopsy failed to provide an adequate explanation, there being no evidence of glycogen storage in the biopsy material and the degree of colitis in two patients who were endoscoped was minimal. The explanation for these radiographic findings is not known at present, although they have similarities to the "cathartic colon syndrome" described in adults.