The aim of the present study was to determine the protein content of the selected five Cucurbitaceae species, analyse their globulin profiles, and assess their potential as source of antidiabetic peptide drugs.
The preponderance of globulins as main reserve proteins in the seeds of Cucurbitaceae was confirmed by the present study. The five species showed globulin content of 52.17 – 295.11 mg/g defatted matter. However, the value obtained for C. lanatus is slightly different from the 228.6 mg/g recorded by Ali et al. working on cultivars from India. The differences noticed may be due to various possible factors like genetic and environmental changes. Likewise, the Cameroonian cultivars are particularly rich in term of globulin diversity, compared to the previous findings by Ali and co-workers, except for Citrullus lanatus where 6 bands were also observed. It is well documented that the nature of reserve protein is directly determined by the nature of enzyme isotype, the expression profile as well as the level of regulation of key enzymes involved in the synthesis and storage. While the nature of the enzyme isotype is intrinsic properties which can be alter by genetic modification, gene expression and regulation might be easily affected by environmental factors [19, 20].
The seeds of T. occidentalis are commonly used in traditional medicine to treat a diversity of diseases including anaemia, convulsion, cardiovascular diseases and liver attack [21, 22]. They are also administered after delivery to stimulate milk secretion in women . The leaves were previously shown to possess hypoglycaemic properties [24–27]. The ethanol extract of leaves, seeds and whole fruits of T. occidentalis were shown to have hypoglycaemic activity . From this study, it was observed that globulins from T. occidentalis, C. lanatus, L. siceraria and C. moschata significantly decreased fasting blood glucose in rates, following induced hyperglycaemia. Nmila et al. showed that Citrullus spp. were rich in phenylalanine and leucine, and possessing insulin-stimulating properties. The globulins investigated in the present study may exert their activity using similar mechanism. Moreover, a peptide with significant biological activity (antifungal) was isolated from T. occidentalis. Blouet  also observed that a mixture of essential amino acids could stimulate insulin secretion irrespective of gastrointestinal factors secreted during the digestion. This effect could be amplified by the digestibility of globulins, since van Loon et al. and Calbet et al. proved that insulin postprandial secretion in response to protein ingestion was influenced by the speed and the amplitude of the apparition of insulin-stimulating amino acids in the plasma. The juice from fruits of another Cucurbitaceae (C. ficifolia), was shown to significantly reduce fasting blood glucose in patients suffering from type 2 diabetes , thereby underlining the potential of Cucurbitaceae as anti-diabetics.
More interestingly, a sharp protein band at 24.61 kDa molecular weight was observed in the profiles of all four globulins profiles with significant anti-hyperglycaemic activity. The band was especially thicker in the species with higher activities, T. occidentalis and L. siceraria; and completely absent in C. mannii (less active). This particular protein which was present is likely to be the active peptide responsible for the activity observed. Further investigations are thus highly needed in order to confirm this hypothesis. The band should be extracted and test separately and the activity compared with both the ones of the other bands, and the whole globulin cocktail of the seeds.