[Properties of sialidases from salivary glands, brain, liver and kidney of the rat].

Sialidase (SD, neuraminidase, EC 3.2.1 18) is an enzyme which releases terminal sialic acid residues from glycoproteins, glycolipids nad oligosaccharides. In this study, we report some characteristics of this enzyme in rat salivary glands, brain, liver and kidney. SD activity was measured fluorometrically by using the synthetic substrate 4-methylumbelliferyl-alpha-D-N-acetylneuraminic acid (4MU-AcNeu). 1. SD activity in tissue homogenates was maximal at pH 4.0 in the submandibular gland (SMG), and at pH 4.5 in both the sublingual gland (SLG) and parotid gland (PG), the values being very similar to those for the activity in brain, liver and kidney. Enzyme activity was highest in SMG, followed by PG and SLG. However, SD activity was lower in these glands than in brain, liver and kidney. 2. Cell fractionation showed that the intracellular distribution of SD in the salivary glands was mainly in the lysosomal fraction, similar to its distribution in brain, liver or kidney. 3. The pH optimum of lysosomal SD was 4.0 in the SMG, and 4.5 in both SLG and PG. The Km value for 4MU-AcNeu of lysosomal SD from all salivary glands was about 0.09 mM. On the other hand, soluble SD in SMG and PG extracts had its pH optimum at 5.5, and a Km value of 0.25mM for 4MU-AcNeu. 4. Studies on heat stability showed SD in the salivary glands and other organs to be very labile, with soluble SD being more labile than lysosomal SD. 5. Lysosomal SD in the PG, brain and liver was inclined to be activated by Ca2+ at low concentrations. Such an effect of Ca2+ was also seen with soluble SD in the SMG, PG and liver. In addition, soluble SD in the SMG was slightly activated by low concentrations of Mg2+. Both Cu2+ and Hg2+ caused a marked inhibition of lysosomal and soluble SD in all organs. 6. By means of Sephacryl S-400 gel filtration, the molecular weight (MW) of lysosomal SD was estimated to be 520,000 in both SMG and SLG, and 460,000 daltons in the PG. The MW of the enzyme in brain, liver and kidney ranged from 350,000 to 460,000. On the other hand, the MW's of soluble SD in SMG and PG were estimated to be 68,000 and 46,000, respectively, by Sephadex G-200 gel filtration. The MW's of soluble SD in brain and liver were very similar to the MW of the corresponding enzyme in the PG.(ABSTRACT TRUNCATED AT 400 WORDS)