Bioassay guided isolation and compounds identification of the anti-diabetic fractions of (rosemary) Rosmarinus officinalis leaves extract

Diabetes mellitus is a metabolic disease characterized by prolonged elevated blood glucose levels. It is a common health problem with a high mortality and morbidity to the human race. A number of medicinal plants such as rosemary (Rosmarinus officinalis) have been used for the treatment of diabetes. Most of the anti-diabetic conventional drugs have been found to have some side effects and there is therefore need to explore new sources of anti-diabetic drugs. The aim of the current study was to investigate the possibility of getting anti-diabetic compounds from R. officinalis that can be used as leads for drug discovery. R. officinalis leaves were macerated in 50% methanol in dichloromethane and the crude extract fractionated by column chromatography. The obtained fractions were subjected to an in-vitro alpha-amylase inhibition assay. The anti-hyperglycemic potential of the fractions was evaluated in diabetic induced Wistar rats. The most potent fractions were analyzed by gas chromatography-mass spectrophotometry (GC-MS) for identification of the compounds. A total of 21 chromatographic fractions were assembled with different alpha- amylase inhibition activity. Eleven of the fractions had more than 30% alpha-amylase inhibition activity. The ethyl acetate fraction had the highest inhibition potential (LC50 of 2.8 μg/mL). The anti-diabetic assay in rats showed that fractions (F1) and (F4) had highest blood glucose reduction of 44.5 ± 0.4 and 52.8 ± 1.3%, respectively (p < 0.05). GC-MS analysis of fractions F1 and F4 showed the presence of 21 and 23 compounds in F1 and 23, respectively. This study has demonstrated that R. officinalis crude extract fractions obtained from 50% methanol in dichloromethane possesses alpha-amylase inhibitory and anti-hyperglycemic activities as well as secondary metabolites with varying chemical structures. The hexane and hexane/ethyl acetate (8/2) fractions showed most potent alpha-amylase inhibition with high anti-hyperglycemic activity giving hope of a possibility of obtaining lead compounds for new anti-diabetic drugs.