Fluorometric assays of different beta-glycosidases in microdissected pancreatic islets from obese-hyperglycemic mice

Cycad toxin-induced damage of rodent and human pancreatic beta-cells

Environmental toxins may be risk factors for some forms of diabetes mellitus and neurodegenerative diseases. The medicinal and food use of seed from the cycad plant (Cycas spp.), which contains the genotoxin cycasin, is a proposed etiological factor for amyotrophic lateral sclerosis/Parkinsonism-dementia complex (ALS/PDC), a prototypical neurodegenerative disease found in the western Pacific. Patients with ALS/PDC have a very high prevalence of glucose intolerance and diabetes mellitus (in the range of 50-80%). We investigated whether the cycad plant toxin cycasin (methylazoxymethanol (MAM) beta-D-glucoside) or the aglycone MAM are toxic in vitro to mouse or human pancreatic islets of Langerhans. Mouse pancreatic islets treated for 6 days with cycasin impaired the beta-cell insulin response to glucose, but this effect was reversible after a further 4 days in culture without the toxin. When mouse islets were exposed for 24 hr to MAM/MAM acetate (MAMOAc; 0.1-1.0 mM), there was a dose-dependent impairment in insulin release and glucose metabolism, and a significant decrease in islet insulin and DNA content. At higher MAM/MAMOAc concentrations (1.0 mM), widespread islet cell destruction was observed. Glucose-induced insulin release remained impaired even after removal of MAM and a further culturing for 4 days without the toxin. MAM damages islets by two possible mechanisms: (a) nitric oxide generation, as judged by increased medium nitrite accumulation; and (b) DNA alkylation, as judged by increased levels of O6-methyldeoxyguanosine in cellular DNA. Incubation of mouse islets with hemin (10 or 100 microM), a nitric oxide scavenger, or nicotinamide (5-20 mM) protected beta-cells from a decrease in glucose oxidation by MAM. In separate studies, a 24 hr treatment of human beta-islet cells with MAMOAc (1.0 mM) produced a significant decrease in both insulin content and release in response to glucose. In conclusion, the present data indicate that cycasin and its aglycone MAM impair both rodent and human beta-cell function which may lead to the death of pancreatic islet cells. These data suggest that a "slow toxin" may be a common aetiological factor for both diabetes mellitus and neurodegenerative disease.

Rapidly induced modulation of beta-N-acetylglucosaminidase activity in pancreatic islets

The aim of this work was to determine the possible rapid modulatory effect of glucose on the activity of pancreatic islet lysosomal enzymes. For this purpose, beta-N-acetylglucosaminidase and beta-galactosidase activities were measured in homogenates of isolated rat islets after a 5, 15, 30 or 60-min exposure to either 3.3 or 16.6 mM glucose. The enzyme activities were determined spectrofluorometrically by means of their respective 4-methylumbelliferyl derivatives as substrates. beta-N-acetylglucosaminidase activity measured in freshly isolated non-incubated islets was 5.482 +/- 0.281 mumol/mg protein/h at 37 degrees C. In islets incubated with 3.3 mM glucose, this activity dropped significantly after 5 min and remained almost constant until the end of the incubation period. In islets incubated with 16.6 mM glucose, beta-N-acetylglucosaminidase activity also decreased significantly at 5 min, and attained its lowest value after 15 min of incubation. After this interval, the activity began to recover and thereafter gained a value close to that measured in non-incubated islets by 60 minutes' time. Despite this ultimate recovery, the enzyme activities measured were significantly lower than those found in islets incubated with 3.3 mM glucose. beta-galactose activity in freshly isolated non-incubated islets was 0.515 +/- 0.094 mumol/mg protein/h at 37 degrees C. This value remained almost unchanged throughout the incubation period in the presence of either 3.3 or 16.6 mM glucose. These results show that beta-N-acetylglucosaminidase activity, a lysosomal hydrolase of pancreatic rat islets,--and only this enzyme--is modulated by glucose.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased serum N-acetyl-beta-D-glucosaminidase and alpha-D-mannosidase activities in obese subjects

We have studied N-acetyl-beta-D-glucosaminidase and alpha-D-mannosidase activities in human sera from 35 control subjects, 47 normo- and hyperinsulinemic obese persons, and 12 diabetic patients after a fasting period of 12 h and at 30, 60, 90, and 120 min after an oral glucose overload. The results show a significantly higher activity of these 2 enzymes in obese subjects and diabetic patients, of similar magnitude, especially in those obese persons with a higher grade of obesity. Moreover, the activity of these glycosidases decreases in a similar way in all these 3 groups after the oral glucose overload.

Biochemical and biophysical characterization of insulin granules isolated from rat pancreatic islets by an iso-osmotic gradient

The size and polydispersity of insulin granules isolated from rat pancreatic islets by centrifugation on a linear iso-osmotic gradient (300 mosM) have been characterized by quasi-elastic light scattering and photon correlation spectroscopy. The separation of granules by the linear gradient technique was compared directly to isolation on discontinuous gradients of hypertonic sucrose (300-1950 mosM) and the greater efficiency of separation assessed by parallel measurements of protein, insulin, cytochrome oxidase and beta-glucuronidase. Granules isolated from pancreatic beta-cells had a mean particle diameter of 342 nm, buoyant density of 1.104, hydrated mass of 23 femtograms and maximal insulin content of 8-9.6 . 10(5) molecules per granule.

Glomerular metabolic changes in compensatory renal hypertrophy

In compensatory hypertrophy, 7 days after unilateral nephrectomy, an increase in respiration and glucose-6-P dehydrogenase activity are observed in the glomeruli of the hypertrophied kidney. It is suggested that is is related to the clearing of an increased amount of filtration residues by the glomerular cells.