Ecto - enzymes

The enzymatic activity in soil is mainly of microbial origin, being derived from intracellular, cell-associated or free enzymes. Only enzymatic activity of ecto-enzymes and free enzymes is used for determination of the diversity of enzyme patterns in soil extracts. Enzymes are the direct mediators for biological catabolism of soil organic and mineral components. Thus, these catalysts provide a meaningful assessment of reaction rates for important soil processes. Enzyme activities can be measured as in situ substrate transformation rates or as potential rates if the focus is more qualitative. Enzyme activities are usually determined by a dye reaction followed by a spectrophotometric measurement. 

The levels of extracellular adenosine could increase step-wise up to micromolar levels as the outcome of the transport and/or diffusion of intracellular adenosine, formed from the large pools of intracellular ATP in hypoxic conditions (Sitkovsky et al. 2005,2008). Hypoxia can upregulate an adenine nucleotide-metabolizing ecto-enzyme cascade comprising ecto-ATP apyrase (CD39) and CD73 (Synnestvedt et al. 2002). 

CD203c ecto-enzyme ectonucleotide pyrophosphatase/phosphodiesterase 3 (E-NPP3) mostly expressed in basofiles... 

Trams, E.G., Lauter, C.J., Salem, N. Jr. and Heine, U. (1981) Exfoliation of membrane ecto-enzymes in the form of micro-vesicles. Biochim. Biophys. Acta 645, 63-70. 

Figure 3. Frequency dependence of electric-field-stimulated ATP hydrolysis activity of a highly purified Ecto nucleotide diphosphohydrolase. The ATP splitting activity of the detergent-solubilized Ecto enzyme was measured at 37 °C in the absence of acfield (a,), in the presence of 5.0-V/cm (peak-to-peak) ac field (O), and in the presence of 10-mM vanadate (a). Buffer 0.1% of detergent NP40, 30-mM histidine, 4-mM ATP, 4-rnM Mg2+, at pH 7.4. The optimal amplitude of stimulation is 5.0 V/cm with a 10-kHz ac field. (Adapted from...

Expression of histochemical markers such as non-specific esterase, lysosomal hydrolases and ecto-enzymes. 

Nucleotidase (EC 3.1.3.5) hydrolyses the phos-phoester linkage in 5 -mononucleotides, liberating a nucleoside and inorganic phosphate. It has been identified as an ecto enzyme (Edelson 1980) in mouse alveolar macrophages (James and Edelson). 

Catalytic site at the external surface (Ecto-Enzymes) ... 

There is very good evidence that several enzymes are ecto-enzymes while others function at the cytoplasmic (internal) side of the membrane. 1 have defined assignments as indefinite or ambiguous where there were conflicting observations, when insufficient evidence was available or when there was a suggestion that the enzyme in question can operate either at the inside or at the outside. Furthermore, individual enzymes may not be found in the plasma membrane of all cells, or conversely, their occurrence in a few cell types constitutes a special case. 

The strongest cases for ecto-enzymes as ubiquitous membrane constituents can be made for the 5 -nucleotidase and for a divalent cation-stimulated nucleoside polyphosphatase (Ca -ATPase). If these two enzymes are not obligatory on the external surface of all eukaryotic cells, then they certainly occur rather commonly as ecto-enzymes. Moreover, they probably occur in several forms, as iso-enzymes, and their properties appear to undergo changes with the physiological or developmental state of the cell (BLQMBERG RAFTELL, 1974 HEWLETT, 1976 ROSENBLATT, 1976). 

There is similar ambiguity in the case of inorganic pyrophosphatase (pyrophosphate phosphohydrolase, EC 3.6.1.1). We had reported that this enzyme appeared to function as an ecto-enzyme in some cultured cells (REICHERT jet, 1974) and similar observations were made elsewhere (STEFANOVIC > 1975a), but the findings... 

Fig. 1. When translocated cytoplasmic ATP impinges on the cell sur ace it is metabolized by a series of ecto-enzymes to adenosine.

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