The topographical location and unique nature of a glucokinase associated with the Golgi apparatus of rat liver

Immunohistochemical Evidence for the Presence of Glucokinase in the Gonadotropes and Thyrotropes of the Anterior Pituitary Gland of Rat and Monkey

A recent report provides new evidence for the presence of glucokinase (GK) in the anterior pituitary. In the present study, immunohistochemistry was used to identify the cells containing GK in the pituitary of rats and monkeys. In rats, GK was detected as a generalized cytoplasmic staining in a discrete population of cells in the anterior pituitary. In colocalization experiments, the majority of cells expressing follicle-stimulating hormone (FSH) or luteinizing hormone (LH) also contained GK. In addition to the gonadotropes, GK was observed in a subpopulation of corticotropes and thyrotropes. GK was not detected in cells expressing growth hormone or prolactin. In monkeys, GK was also observed in a discrete population of cells. Intracellular distribution differed from the rat in that GK in most cells was concentrated in a perinuclear location that appeared to be associated with the Golgi apparatus. However, similar to rats, colocalization experiments showed that the majority of cells expressing FSH or LH also contained GK. In addition to the gonadotropes, GK was observed in a subpopulation of corticotropes and thyrotropes. In the monkey, only a few cells had generalized cytoplasmic staining for GK. These experiments provide further evidence for the presence of GK in the anterior pituitary. Although some corticotropes and thyrotropes contained GK, the predominant cell type expressing GK was gonadotropes. In view of the generally accepted role of GK as a glucose sensor in a variety of cells including the insulin-producing pancreatic β-cells as the prototypical example, it is hypothesized that hormone synthesis and/or release in pituitary cells containing GK may be directly influenced by blood glucose.

N-acetylglucosamine-insensitive glucose phosphorylation in isolated hepatocytes from rats fasted for 48 h

Glucose phosphorylation in isolated hepatocytes was studied by the release of 3H from D-[2-3H]glucose. Glucokinase activity was decreased by fasting rats for 48 h and was further reduced in cells by adding 30 mM GlcNAc, a potent competitive inhibitor. Although this treatment resulted in the loss of more than 97% of glucokinase activity in hepatocytes, glucose phosphorylation proceeded at an appreciable rate. These observations demonstrate the involvement of a high -K0.5 enzyme system in addition to glucokinase in hepatocyte glucose phosphorylation.

Regulation of glucokinase activity in the domestic fowl

The factors which regulate soluble and particulate glucokinase and hexokinase activity in the liver of domestic chickens has been investigated. Pretreatment with oral administration (via tube feeding) of glucose plus injection of insulin resulted in a significant increase in the activity of soluble (p less than 0.01) and particulate (p less than 0.01) glucokinase activity whereas fasting for 48 hours reduced glucokinase and hexokinase activity (p less than 0.01) in the particulate fraction only. Treatment of fed chickens for 2 weeks with thyroxine (50 micrograms: i.m. daily) plus triiodothyronine (50 micrograms) resulted in a marginal decrease (NS) in soluble glucokinase activity but significantly increased soluble hexokinase (p less than 0.05) activity. Thyroidectomized animals showed a decline in both soluble glucokinase (p less than 0.01) and hexokinase (p less than 0.025) activity. There was no effect of thyroid hormone manipulation on particulate glucokinase activity although there was a significant reduction in particulate hexokinase activity (p less than 0.05) in thyroidectomized birds. These data establish a physiological role for the glucokinase enzyme activity in avian carbohydrate metabolism and suggest that in contrast with the mammal, the particulate fraction is the more physiologically important enzyme.

Activities of glucokinase and hexokinase in mammalian and avian livers

A radiochemical assay for glucokinase activity was developed for use in high-speed supernatants of liver. The maximum activities of glucokinase ranged from 0.4 to 3.8 mumol/min per g fresh wt. at 30 degrees C in some avian and mammalian livers, including pigeon, guinea pig and man, in which previous reports indicated zero activities. The reported maximum rates of hepatic glycogen synthesis in livers of rat and man in vivo are similar to the calculated glucokinase activities at 10mM-glucose; therefore glucokinase activity should not limit glycogen synthesis from glucose.

Distribution of newly synthesized DT-diaphorase in rat liver

The distribution, synthesis, transport, and glycosylation of rat-liver DT-diaphorase has been investigated. The enzyme could be isolated using specific antibodies, mainly from the soluble supernatant but also from microsomal vesicles, Golgi membranes, and mitochondria. 40% of the microsomal enzyme was located in the lumen or on the interior side of the membrane, the rest remaining as an integral non-extractable part of the membrane. Synthesis of DT-diaphorase takes place on both free and bound ribosomes, although it was found to be transported in a sequential manner from the rough to the smooth endoplasmic reticulum and also subsequently to the mitochondria. The rough and smooth microsomal DT-diaphorase contains covalently bound carbohydrate, but no sugar moiety could be detected bound to the cytoplasmic form of the enzyme.

[Liberation and reassociation of the microsomal membrane glucokinase in cat liver]

The particulate glucokinase of cat liver is shown to be microsomal. The activity is readily solubilized by glucose-6-phosphate, ATP, pyrophosphate, high salt concentrations and, to a lesser extent, ribonucleoside triphosphates. The solubilization by glucose-6-phosphate is inhibited by Pi. Solubilizations by ATP and glucose-6-phosphate differ in their sensitivity to temperature changes; they are relatively specific for glucokinase as compared to solubilization by detergent (Triton X 100). The enzyme can be bound again to previously eluted microsomal membranes. Treatment of membrane with trypsin, at 0 degrees C, destroys the ability to rebind the enzyme to the membrane. It is suggested that electrostatic forces are of considerable importance for the binding of glucokinase to a possible protein binding site in the membrane.

[Presence of a pathway for biosynthesis of UDP-glucose in the Golgi apparatus of rat hepatocytes]

Glucokinase, phosphoglucomutase and glucose-1-phosphate uridylyltransferase are the three enzymes involved in a microsomic pathway for the synthesis of UDP glucose. Evidence is given, in this paper, for the localization of these three enzymes in a Golgi-rich fraction of rat liver. This fraction is prepared, from smooth microsomes, by the means of a discontinuous four-step sucrose gradient. Three of the lighter fractions (d = 1.08-1.13) are enriched in the Golgi markers (galactosyltransferase, sialytransferase and thiamin pyrophosphatase), especially the one with density 1.13. The three enzymes we are interested in are enriched in the two upper hands (d 1.08-1.11), which display an activity for the biosynthesis of UDP-glucose from glucose equivalent to the one obtained in a crude microsomic preparation, and which are not contaminated by other subcellular components.