Characterisation of a unique dipeptidase allele in an insular population of masked shrews, Sorex cinereus (Insectivora: Soricidae)
2003
Macpherson, I.S. | Stewart, D.T.
The masked shrew, Sorex cinereus, is found in extremely high densities on Bon Portage Island, off the southern coast of Nova Scotia, Canada. This population is sustained by abundant amphipods and kelp flies living in large beds of decaying seaweed. Littoral feeding has not been demonstrated in other Sorex cinereus populations. This population is also distinct in that it contains a unique allele for a cytosol non-specific dipeptidase (peptidase A, E.C. 3.4.13.18). This allele does not appear to be present elsewhere in the region, even in an adjacent mainland location. The co-occurrence of a unique diet and high frequency of a unique form of a digestive enzyme begs the question of whether or not this is an adaptive genetic variant. As a first step towards testing this hypothesis, we developed a protocol for isolating and characterising this enzyme. Despite the extremely small size of these shrews (approximately 3 g), a technique was developed to produce a reasonably pure extract from the small intestine (< or = 100 mg). The activity of the enzyme was then measured using a trinitrobenzene sulfonic acid based assay. pH optimum assays showed that both the mainland allele and island dipeptidase alleles have similar pH optima of 8.0+/-0.1. This value is similar to that observed for other mammalian species.
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