Comment to Sherr and Sherr (1999): "Is there any appropriate way to distinguish different beta-N-acetylhexosaminidase activities in aquatic environments?"

The recent paper of Sherr and Sherr on detecting low-affinity beta-glucosam inidase activity in several marine microbes extends current knowledge about hydrolytic enzyme activities in natural aquatic systems. However, their co nclusions regarding the whole-cell assay with MUF-N-acetyl-beta-D-glucosami nide (MUF-[GlcNAc]) cannot be accepted. First, we explicitly demonstrate a strong correlation between extracellular activities of the high-affinity en zymes and grazing rates of bacterivorous protists. Therefore, the assay can still be recommended for the estimation of total protistan grazing on prok aryotic picoplankton. Second, the ability of many aquatic organisms to prod uce enzymes which cleave fluorogenic substrates, such as MUF-[GlcNAc] and/o r MUF-beta-D-N,N',N"-triacetylchitotriose (MUF-[GlcNAc](3)), has been well- documented during the last decade. Thus, neither of the two substrates may be considered as exclusively specific for targeting either lysozymes or bet a-N-acetylhexosaminidases. (C) 2000 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.