Targeted metabolomics of organic and amino acids in giraffe milk during mid- to late-lactation

Background At the end of lactation (mammary involution), dynamic changes in milk components occur in all mammals. The time to reach complete cessation varies among taxa and species. The involution of cows, sheep, and goats (Bovidae) has been reported, but limited information is available on giraffes. Objectives Characterise organic acids and amino acids in the milk of giraffes at involution. Methods Milk was obtained from five giraffes. A LC-MS/MS metabolomics approach was followed, and statistical analysis of the data was done using MetaboAnalyst 6.0. Results There were 38 organic acids and 45 amino acids measured in the giraffe milk. The organic acids indicate a decrease in Krebs cycle intermediates. Lower citrate levels are associated with lower lactose levels, indicating reduced osmotic regulation. Lower uracil and orotic acid indicate decreased pyrimidine synthesis and eventual nucleotide synthesis. Increased amino acid content is not devoted to protein synthesis, but to other functions, specifically as antioxidants, redox buffering, and cytoprotection. Increased histidine, serine and methionine promote protein degradation and one-carbon metabolism. Lysine catabolites lead to decreased levels of energy metabolites and to stress adaptation. Aromatic amino acids modulate the supply of immune and neuroactive metabolites. Conclusions During involution, the regulation of organic acids suggests reduced Krebs cycle activity, indicating a transition from high biosynthetic to catabolic activity. Likewise, amino acids have other functions, specifically antioxidant, redox-buffering, and cytoprotection.