A Biochemical study on the types of digestive specific proteases in the ladybird, Cryptolaemus montrouzieri Mulsant (Col.: Coccinellidae) and the effects of different preys on their activity

Type and biochemical properties of the digestive proteases were determined using specific substrates and inhibitors in the larvae and the adults of Cryptolaemus montrouzieri and their activity changes were evaluated when fed on the different preys. Rearing of beetles was done in laboratory conditions and they were fed on citrus mealybug. Dissection was done in NaCl medium and the obtained midgut was homogenized and centrifuged. Then, the activity of digestive proteases determined by using the specific substrates and inhibitors by feeding on three hosts. The results showed that just cysteine proteases and exopeptidases were active in the midgut of adults while both serine and cysteine proteases were engaged in the digestion by the larvae. The highest activity of the serine proteases; trypsin, chymotrypsin and elastase, were found in the pH sets of 8, 8-9 and 9, respectively. The optimal pH of cathepsins B and L were determined in 6 and 5 pH values of the larvae and the adults of C. montrouzieri, respectively. The highest activities of digestive larval amino- and carboxypeptidases were measured at pH 7 while these two enzymes showed the highest activity at pH sets of 7 and 6 in the adults, respectively. The specific inhibitors including cystatin, E-64, TLCK, SBTI, TPCK and AEBSF.HCL significantly decreased the activities of specific proteases in the larvae and the adults of C. montrouzieri. No significant differences were observed in the activities of serine proteases, exopeptidases and cathepsin L in the larvae fed on Planococcus citri, Pseudococcus viburni and Toxoptera aurantii but cysteine proteases and aminopeptidases in the adults fed on tea aphid showed the least activity. The difference in proteolytic profile of the larvae and the adults of C. montrouzieri concur the physiological differences in these two developmental stages with the emphasize on serine proteases in the preservation and control of developmental status of digestive tract. In addition, the higher activity of cysteine proteases in the adults may be related to the nature of utilized prey and presence of different proteinaceous components or even glycoperoteins in the preys.