Inhibitory effects of aloe carboxypeptidase fraction on streptozotocin-induced enhancement of vascular permeability in the pancreatic islets

Abstract

The protective actions of components isolated from Aloe arborescens Miller var. natalensis Berger (Kidachi aloe in Japanese) on streptozotocin (Sz)-induced necrosis of B cells in the pancreatic islets of the mouse were investigated to clarify its action mechanism involved in anti-diabetic effects. In this experiment, phenol low molecular weight components of aloin and aloin A that were anti-oxidants and derived from the leaf skin or pulp extract, an aloe carboxypeptidase fraction that is a inhibitor of enhanced vascular permeability and a glycoprotein component that decreases blood glucose were tested with mice precedently administered with Sz which is known as a cytotoxin specific to B cells. The results showed that the treatment group receiving Sz followed by the aloe carboxypeptidase fraction increased the inhibition of dye leakage by 75.8% ($p<0.001$) in the extract of whole pancreas in comparison to the control group and the aloe carboxypeptidase fraction group also increased the inhibition effect by 68.4% ($p<0.001$) in the extract of pancreatic islets as compared to the control group. The carboxypeptidase is an aloe-derived protease known to inhibit the acetic acid-related enhancement of intraperitoneal vascular permeability in mice. Further, the elevation of blood glucose in Sz-induced diabetic mice intraperitoneally given the aloe carboxypeptitase fraction was significantly ($p<0.01–0.001$) restrained at 3, 7 and 14 days after the injection as compared to the control group given solvent only. The results of this experiment suggested that the inhibitory effect on the enhancement of vascular permeability related to the vascular acute inflammatory response at Sz-induced lesions of pancreatic islets was involved in the action mechanism of this enzyme.

Introduction

There are plenty of articles related to the anti-diabetic effects of components that are of plant origin (Roman-Ramos et al., 1995; Alarcon-Aguilara et al., 1998; Reynolds and Dweck, 1999; Yeh et al., 2003; Eddouks et al., 2003). Polysaccharides like panaxan A, aconitan A and ganoderan B and C that exhibited hypoglycemic effects were found in Japan from galenicals such as Panax ginseng, Aconitum carmichaeli, Ganoderma lucidum (Takahashi et al., 1985; Konno et al., 1985a, Konno et al., 1985b; Tomoda et al., 1985, Tomoda et al., 1987, Tomoda et al., 1990; Hikino et al., 1985, Hikino et al., 1989). These active polysaccharides were found to have a structure composed of glucan (Tomoda et al., 1984, Tomoda et al., 1986a, Tomoda et al., 1986b), and their actions were reportedly to increase blood concentration of insulin and decrease glycogen in the liver of mice (Hikino et al., 1989).

On the other hand, there are several publications relevant to Aloe barbadensis Miller (Aloe vera Linne) and Kidachi aloe (Ghannam et al., 1986; Ajabnoor, 1990; Beppu et al., 1993; Chithra et al., 1998; Okyar et al., 2001), and Acemannan, glycoprotein and polysaccharides were cited as active ingredients. We have isolated hypoglycemic components from the mesophyll of Kidachi aloe that is a good source of polysaccharides (Beppu et al., 1990). Meanwhile, it was confirmed that in the leaf skin, there were some components exhibiting inhibitory actions of hyperglycemia as a result of the prevention of necrotic inflammation of pancreatic islet B cells (Beppu et al., 1990).

Streptozotocin (Sz) specifically destroys pancreatic islet B cells (Junod et al., 1967; Like et al., 1978; Agarwal, 1980; Cooperstein and Watkins, 1981). It was detected in the pancreatic islets of mice and rats that dye leakage related to increased vascular permeability reached a peak 4–8 h after Sz injection, when Monastral blue B or Evans blue was serially and intravenously injected (Sandler and Jansson, 1985; Beppu et al., 1987).

Majno et al. (1987) confirmed, using BB rats in Worcester colony (hereinafter referred to as “BB/Wor”) of human insulin-dependent diabetes rat model, that the development of pancreatic vascular leakage (PVL) was intensified by the injection of Monastral blue B of a colloidal pigment into the tail vein of 20–200 days old BB/Wor rats. Nakano (1991) reported that PVL developed before insulitis manifested in BB/Wor rats. Majno and Doukas group found that intravenous injection of purified immunoglobulins from RT6-depleted diabetes-resistant (DR)-BB rat induced abnormal PVL in mice. The presence of immunoglobulins early in the disease process and their ability to induce PVL would suggest that they might participate in one of diabetes pathogenesis. More specifically, the pathomorphism of PVL would induce insulitis to be one of pathogenesis of diabetes mellitus (Doukas et al., 1996).

Now therefore, it is expected that the restraint of progressing PVL could retard a chain reaction of insulitis and the development of diabetes mellitus. The inhibition of vascular permeability in pancreatic islets may reduce the destruction of the pancreatic islets and retard the elevation of blood glucose, thus preventing the development of diabetes.

Previously, we pathologically found that a component of Kidachi aloe leaf skin showed more potent protective actions on B cell necrosis induced by Sz, which is known as a cytotoxin specific to B cells, than a component of Kidachi aloe leaf pulp.

We administered leaf skin and pulp extract components derived from a phenol low molecular weight component, a glycoprotein component and a carboxypeptidase fraction derived from Kidachi aloe to mice to compare the inhibitory effects on the Sz-induced enhancement of vascular permeability so that we can search any component participated in other action mechanisms to protect pancreatic islet B cells than the hypoglycemic action shown by glucans, Acemannan or other polysaccharides.