Studies on kidney sialidase in normal and diabetic rats

Abstract

Rat kidney cortex sialidase was studied using α- sialyl-(2→3)-[ 3 H] lactitol and α- sialyl-(2→6)-[ 3 H] lactitol as substrates. The enzyme was found mainly in the lysosomal fraction. Only 23% of the sialidase activity of this fraction could be solubilized by a combination of freezing-thawing, sonication and Triton X-100 treatment. The optimal pH for the lysosomal enzyme activity was 4.2 and the enzyme's K m values for αsialyl-(2→3)-lactitol and α-sialyl-(2→6)-lactitol were 0.28 and 0.41 mM, respectively. The specific activity was twice as high with the former substrate than with the latter. Sialidase activities in dialyzed kidney cortex homogenates of streptozotocin-diabetic rats and of age-matched control rats were compared. The specific activity was found to be significantly increased in the diabetic animals when using both substrates 5950±720 (S.E.) dpm/h per mg protein ( n=7) vs. 3970±370 in the controls ( n = 8 with α-sialyl-(2-→3)-lactitol ( P <0.025) and 2870±300 vs. 1820±170 with α-sialyl-(2→6)-lactitol ( P<0.02). The activities were also found to be increased when expressed per whole kidney cortex ( P<0.005 and P<0.001, respectively. The elevated sialidase activity in diabetic kidney cortex may be related to the reported decrease in sialic acid content of the glomerular basement membrane, which lowers its negative charges and which may contribute to an increased permeability to proteins.