Assessment and validation of methods for the determination of γ-glutamyltransferase activity in sheep milk

The determination of enzymatic activity in sheep milk is still today a practically unexplored field of research. This study proposes and fully validates two analytical procedures (i.e. by means of UV–vis spectrophotometry and RP-HPLC methods) for the determination of c-glutamyltransferase activity in sheep milk. Both methods are characterised by low detection and quantification limits, excellent linearity over a wide enzymatic activity interval, very good repeatability and reproducibility, and are bias-free. The RPHPLC method provides better sensitivity and automation capability levels than that of UV–vis, and its use is hence suggested for screening purposes. These methods have been preliminarily tested with a number of real samples of whole sheep milk, obtaining an average c-glutamyltransferase activity value of 2.93 ± 0.50 U ml1 and a range from 2.72 ± 0.10 U ml1 to 3.46 ± 0.11 U ml1.