Altered glycosylation of gamma-glutamyltranspeptidase (GGT) in seminal fluid from men with accessory gland infection

Quantifying proteins by mass spectrometry: the selectivity of SRM is only part of the problem

Precise and accurate protein quantification is critical to many areas of proteomics. Antibody-based approaches are costly and time-consuming to develop, consequently, there is considerable interest in alternative quantitative methods that are versatile and can be implemented without the considerable delays associated with antibody development and characterization. Approaches based on MS have therefore attracted considerable attention and are now frequently touted as the most practical and powerful of all options. Nevertheless, there are serious limitations associated with quantifying a protein based on tandem mass analysis of one or two peptides generated by either chemical or enzymatic cleavage. In an accompanying Viewpoint article, Molloy and coworkers point out that selectivity is not necessarily guaranteed despite the power of SRM. Here we address an additional concern that can also compromise specificity. In complex mammalian systems, multiple proteins can serve as precursors of a single peptide and consequently, depending on the peptide(s) selected, protein levels may be significantly under- or overestimated.

Activation energy and lectin affinity chromatography of gamma-glutamyltransferase as a marker for enzyme heterogeneity

Lectin affinity chromatography of gamma-glutamyl transferase (GGT,EC 2.3.2.2) is able to detect differences in the carbohydrate moiety of the enzyme. Binding of tissue GGT towards lectins is significantly different from serum GGT, showing increased galactosylation in tissue forms. Kidney GGT is less glycosylated than GGT from other tissues (liver, pancreas, prostate, vesiculae seminales). Increases in sialic acid content of GGT are associated with an increase in the activation energy of the catalyzed reaction. Differences in galactose, fucose and N-acetylhexosamine content induce much smaller effects on activation energy. In liver diseases, serum GGT is characterized by an altered affinity against lectins recognizing galactose, fucose and N-acetyglucosamine and by increased activation energy. In patients with liver disease, use of fixed temperature conversion factors can lead to erroneous calculations of serum GGT enzyme activity (errors up to 13.3%).

Measurement of activation energy of gamma-glutamyltransferase as a marker for enzyme heterogeneity

In order to detect differences between various multiple forms of gamma-glutamyltransferase, the activation energy was measured. In the serum of patients with liver diseases, activation energy was measured. In the serum of patients with liver diseases, activation energy of the serum enzyme is higher than in normal individuals (41.9 +/- 1.2 vs. 38.9 +/- 1.5 kJ/mol, p less than 0.05). Neuraminidase treatment resulted in a reduction of activation energy. Various multiple forms of serum gamma-glutamyltransferase, as prepared by lectin affinity chromatography (concanavalin A, Ricinus communis I and II, wheat germ agglutinin) showed activation energy differences between binding and nonbinding fractions. Similar results were observed in seminal plasma gamma-glutamyltransferase, when patients with accessory gland infection were compared with a reference population. Our results suggest that the activation energy depends upon differences in the carbohydrate part of the enzyme. The low gamma-glutamyltransferase activation energy of tissue extracts increased significantly after butanol extraction and was then comparable with serum activation energy values, which suggests that lipid-binding is a factor in activation energy variation. In most cases, gamma-glutamyltransferase activities measured at a certain temperature can be easily converted to a corresponding activity at another temperature, but in severe liver disease significant errors may be introduced when simple temperature conversion factors are used.

Lectin-affinity chromatography of serum gamma-glutamyltransferase in liver disease

The variation of the carbohydrate chain of gamma-glutamyltransferase was studied in 45 liver patients by means of lectin affinity chromatography. Five lectins were used: concanavalin A, Ricinus communis I and II, Maclura pomifera and Ulex europaeus agglutinin. The binding towards Con A was shown to be independent from the binding towards the other lectins. Parallel variations of binding results against the galactose- and fucose-recognizing lectins were obtained. In liver steatosis, the binding results were comparable to those obtained in normal patients. Cirrhosis and metastasis patients showed a decreased binding towards Con A, while the binding against the various galactose- and fucose-recognizing lectins was increased. After neuraminidase treatment, an increased affinity towards all lectins was observed. However, differences in RCA I and RCA II binding between patients and controls still persisted. Besides sialic acid, also galactose and fucose residues contribute to serum gamma-glutamyltransferase heterogeneity.