Comparing methods for protein analysis of arthropods

The protein content of arthropods can be an important measure of their nutritional quality and can influence the flow of nutrients through food webs. Protein content is often measured through surrogates, including crude protein (nitrogen x 6.25) or protein assays (Bradford, Lowry, BCA). Yet, it remains unclear which surrogate of protein is most closely related to the actual protein content of arthropods. We measured protein content of a diversity of arthropods using hydrolyzed amino acid analysis and compared these results with crude protein and protein assay measurements of the same samples. Among 13 orders of arthropods, the Bradford and Lowry assay measurements were strongly correlated with metabolizable protein content. However, when we remove one outlier, German cockroaches, then the crude protein content of non-exoskeleton tissue was the best predictor of metabolizable protein content. German cockroaches have high nitrogen content (i.e., 15.2 % by dry mass) likely because they can store nitrogen as uric acid, which appears to be rare among arthropods. We also examined within-order relationships between protein content and its surrogates in Coleoptera, Hemiptera, and Orthoptera. Crude protein of non-exoskeleton tissue was strongly correlated with metabolizable protein content in all three orders. Incorporating data on exoskeleton content of arthropods into the crude protein measure improved its ability to predict protein content within and among orders of arthropods. Our results suggest that some protein assays and an improved measure of crude protein can be reasonable surrogates for the protein content of arthropods, depending on the types of samples being analyzed.