Detection of aminotransferase activity of Propionibacterium freudenreichii after SDS-PAGE Anne Thierry * , Marie-Bernadette Maillard, Sylvie Lortal Laboratoire de Recherches de Technologie Laitie `re, I.N.R.A., 65 rue de Saint-Brieuc, 35042 Rennes Cedex, France Received 12 November 2001; received in revised form 19 March 2002; accepted 19 March 2002 Abstract Aminotransferases (ATs) had previously been detected after native electrophoresis. We show now that aminotransferase(s) of Propionibacterium freudenreichii can be detected after sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS- PAGE). Moreover, it retained a high activity (84%) in the presence of 0.23% SDS, contrary to what was observed for aminotransferase(s) of Bifidobacterium bifidum (54%) and of six other cheese-related species (0 – 20%). D 2002 Elsevier Science B.V. All rights reserved. Keywords: Aminotransferase; Cheese-related bacteria; Propionibacterium; SDS-PAGE; Zymogram 1. Introduction Numerous enzymatic activities have been detected in zymograms, after nondenaturing polyacrylamide gel electrophoresis (PAGE) (Gabriel and Gersten, 1992). However, some of these activities can also be detected after sodium dodecyl sulphate (SDS)-PAGE (Bischoff et al., 1998; Soler et al., 1999). Aminotransferases (ATs) are ubiquitous enzymes involved in amino acid synthesis and catabolism. They catalyse the conversion of amino acids to the corresponding a-ketoacids, in the presence of an a- ketoacid acceptor of amino group (for example, a- ketoglutaric acid) which is converted into amino acid (glutamic acid). ATs of cheese-related bacteria have recently received an increase in interest, as they are responsible for the first step of degradation of amino acids to aroma compounds during cheese ripening. Nondenaturing zymograms have been used to detect AT activity in Lactococcus lactis (Gao et al., 1997; Yvon et al., 1997), as previously demonstrated for various plants (Driedger et al., 1994; Yamamoto and Duich, 1994). Since the use of a denaturing agent such as sodium dodecyl sulphate (SDS) improves resolu- tion and allows molecular mass estimation, we assayed the detection of AT activity after SDS-PAGE. For that purpose, we determined first the effect of SDS toward AT activities in the cell-free extracts (CFE) of several cheese-related bacteria, and then directly after SDS-PAGE. 2. Material and methods 2.1. Strains and preparation of cell-free extracts CFE were prepared of ten strains grown to late log phase on reference media (Table 1). Cells were 0167-7012/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII:S0167-7012(02)00060-X * Corresponding author. Tel.: +33-2-23-48-53-37; fax: +33-2- 23-48-53-50. E-mail address: athierry@rennes.inra.fr (A. Thierry). www.elsevier.com/locate/jmicmeth Journal of Microbiological Methods 51 (2002) 57 – 62