Tryptophan, Kynurenine and Kynurenic Acid Concentrations in Milk and Serum of Dairy Cows with Prototheca Mastitis

Simple Summary Bovine mastitis continues to be a leading cause of heavy economic losses in the dairy industry worldwide. Among a broad spectrum of infectious agents implicated in this pathology are unicellular, achlorophyllous microalgae of the genus Prototheca. The prevalence of mastitis due to Prototheca algae is currently increasing. The aim of this work was to explore a possible association of the kynurenine pathway of tryptophan metabolism with Prototheca mastitis. The authors were eager to know whether metabolites of tryptophan degradation can be used as markers of the protothecal bovine mastitis. Both tryptophan and its metabolite kynurenine occurred at a significantly lower level in the milk of cows with mastitis compared to healthy animals. Whereas the activity of indoleamine 2,3-dioxygenase, a key enzyme of tryptophan catabolism, was significantly higher in milk of mastitic cows compared to control animals. It was thus concluded that low values of tryptophan and kynurenine concentrations or elevated indoleamine 2,3-dioxygenase activity in milk samples can be used as markers of mastitis due to Prototheca spp. Abstract The aim of this work was to investigate serum and milk levels of tryptophan (TRP), kynurenine (KYN), and kynurenic acid (KYNA), as well as the activity of indoleamine 2,3-dioxygenase (IDO) in cows with mastitis due to Prototheca algae. The study was prompted by previous research showing a link between the KYN pathway of TRP metabolism and bovine mastitis of bacterial etiology. The study was carried out over a 2-year period (2018–2019) and included quarter milk and serum samples collected from six dairy herds in Poland. The samples were obtained from healthy cows and cows with Prototheca mastitis of either clinical and subclinical manifestation, as determined upon direct measurement of the somatic cell count or indirectly by performing a California Mastitis Test on suspected quarters. Both TRP and KYN concentrations were significantly lower in milk of mastitic cows compared to healthy animals (0.8 vs. 8.72 µM, p = 0.001; 0.07 vs. 0.32 µM, p = 0.001, respectively). The difference in TRP and KYN concentrations in the sera of the two animal groups was much less pronounced (25.55 vs. 27.57 µM, 3.03 vs. 3.56 nM, respectively). The concentration of KYNA was almost at the same level in milk (1.73 vs. 1.70 nM) and in serum (80.47 vs. 75.48 nM) of both mastitic and healthy cows. The data showed that the level of TRP and its metabolites in serum was conspicuously higher compared to milk in all cows under the study. The activity of IDO was significantly higher in milk of cows with Prototheca mastitis compared to healthy animals (71.4 vs. 40.86, p < 0.05), while in serum it was pretty much the same (135.94 vs. 124.98, p > 0.05). The IDO activity differed significantly between serum and milk both for mastitic (135.94 vs. 71.4, p < 0.05) and healthy cows (124.98 vs. 40.86, p < 0.001). In conclusion, low values of TRP and KYN concentrations or elevated IDO activity in milk samples might be used as markers of mastitis due to infectious causes, including Prototheca spp.