1. The activities of 30 different lysosomal enzymes were determined in vitro in the presence of the sulphated glycosaminoglycans, heparin and chondroitin sulphate, all the enzymes being measured on a density-gradient-purified lysosomal fraction. 2. Each enzyme was studied as a function of the pH of the incubation medium. In general the presence of sulphated glycosaminoglycans induced a strong pH-dependent inhibition of lysosomal enzymes at pH values lower than 5.0, with full activity at higher pH values. However, in the particular case of lysozyme and phospholipase A2 the heparin-induced inhibition was maintained in the pH range 4.0-7.0. 3. For certain enzymes, such as acid β-glycerophosphatase, α-galactosidase, acid lipase, lysozyme and phospholipase A2, the pH-dependent behaviour obtained in the presence of heparin was quite different to that obtained with chondroitin sulphate, suggesting the existence of physicochemical characteristic factors playing a role in the intermolecular interaction for each of the sulphated glycosaminoglycans studied. 4. Except in the particular case of peroxidase activity, in all other lysosomal enzymes measured the glycosaminoglycan-enzyme complex formation was a temperature-and time-independent phenomenon. 5. The effects of the ionic strength and pH on this intermolecular interaction reinforce the concept of an electrostatic reversible interaction between anionic groups of the glycosaminoglycans and cationic groups on the enzyme molecule. 6. As leucocytic primary lysosomes have a very acid intragranular pH and large amounts of chondroitin sulphate, we propose that this glycosaminoglycan might act as molecular regulator of leucocytic activity, by inhibiting lysosomal enzymes when the intragranular pH is below the pI of lysosomal enzymes. This fact, plus the intravacuolar pH changes described during the phagocytic process, might explain the unresponsiveness of lysosomal enzymes against each other existing in primary lysosomes as well as its full activation at pH values occurring in secondary lysosomes during the phagocytic process.

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