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Homocysteine, namely 2-Amino-4-mercaptobutyric acid, which is a sulfur containing amino acid, is a product of methionine metabolism. Recently people increasingly realized that the homocysteine (Hcy) levels in cells and serum of human body have relationship with health. It has now been widely accepted that hyperhomocysteinemia is a major independent risk factor for cardiovascular disease. Therefore the determination of homocysteine is very important for clinical diagnosis of cardiovascular disease. At present, the methods generally to be used for the determination of homocysteine in human plasma are high-performance liquid chromatography (HPLC) and fluorescence polarizationimmunoasssay (FPIA). However, in view of the time and cost consuming, at present it is used only in some big hospitals and test centers. So that the enzyme colorimetry assay, which is much cheapper and simpler than those ones, is a very promising method for the determination of homocysteine.The homocysteine y-lyase (desulfhydrase) (EC 4.4.1.2 ) is a important enzyme to be used in the enzyme colorimetry method. Consequently the properties of it are essential for the practical application of this method. At present, also some researches of homocysteine y-lyase had been reported, in most cases, the enzymes’ characteristics are not very well.In this study, a strain of bacteria capable of producing homocysteine y-lyase was isolated and which was identified subsequently as a strain of the genus Serratia. In this work the conditions of the production, as well as the characteristics of the crude enzyme were studied. The results were showed as follows:1. The isolation and identification of the homocysteine y-lyase producing strain: Five cultures of bacteria, which were strain N2-1, N4-1, N8-2, N10 and N12-2, capable of producing homocysteine y-lyase were isolated from the soil. Based on the homocysteine y-lyase activity and specificity for homocysteine, the strain N2-1, which showed highest homocysteine y-lyase activity and specificity, was selected. On the basis of its morphological, physiological and biochemical characteristics, as well as 16S rDNA sequence and phylogenetic tree analysis, this new isolate was assigned to the genus
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