Determinations of dimethylsulphoniopropionate (DMSP) lyase activity using headspace analysis of dimethylsulphide (DMS)

The osmolyte dimethylsulphoniopropionate (DMSP) can be enzymatically cleave d to dimethylsulphide (DMS), acrylate and a proton. The enzyme involved in this reaction is dimethylpropiothetin dethiomethylase (DMSP lyase; enzyme c lassification number 4.4.1.3.). Although the importance of this reaction fo r the global sulphur cycle, the influence of DMS on atmospheric acidity and the possible effect on climate regulation have been widely recognised, our knowledge of DMSP lyases is limited to just a few studies. Activity measur ements of DMSP lyases offer an important step towards a better understandin g of the conditions under which DMS is produced. In the available published data somewhat similar methods have been used before, but a critical examin ation of the method limitations has not been reported. To encourage further research on this enzyme, we suggest and detail two protocols for measureme nts of DMSP lyase activity: An in vitro assay for crude cell extracts or pu rified enzyme and an in vivo method for whole cells, which we recently star ted to use. After addition of DMSP, samples incubated in a gas tight vial m ay produce DMS from enzymatic cleavage under suitable conditions, and a DMS production rate can be estimated from time-series measurements of DMS in t he headspace of the vial. Headspace analysis of DMS is a useful and rapid t echnique to estimate and compare DMSP lyase activities from different sourc es. The relative rates of DMS production in the liquid and of the gas trans fer between liquid and headspace, determine the rate of DMS production meas ured via headspace analysis. If DMS production in the liquid is higher than the rate of transfer, headspace measurements will not reflect the actual a mount of DMS produced in the liquid. In this case, extracts have to be dilu ted to a level that ensures linearity between dilution factor and reduction of enzyme activity. Additionally, incubation Volumes and vials should be s elected to provide a high surface-to-volume ratio to ensure maximum flux of DMS from the aqueous phase into the headspace. The methods can be adapted to further investigate species- and strain-specific activities, biogeograph ical distribution, cellular location and biochemical properties of various DMSP lyases. (C) 2000 Elsevier Science B.V. All rights reserved.