We introduce neutron-encoded (NeuCode) amino acid labeling of mice as a strategy for multiplexed proteomic analysis in vivo. Using NeuCode, we characterize an inducible knockout mouse model of Bap1, a tumor suppressor and deubiquitinase whose in vivo roles outside of cancer are not well established. NeuCode proteomics revealed altered metabolic pathways following Bap1 deletion, including profound elevation of cholesterol biosynthetic machinery coincident with reduced expression of gluconeogenic and lipid homeostasis proteins in liver. Bap1 loss increased pancreatitis biomarkers and reduced expression of mitochondrial proteins. These alterations accompany a metabolic remodeling with hypoglycemia, hypercholesterolemia, hepatic lipid loss, and acinar cell degeneration. Liver-specific Bap1 null mice present with fully penetrant perinatal lethality, severe hypoglycemia, and hepatic lipid deficiency. This work reveals Bap1 as a metabolic regulator in liver and pancreas, and it establishes NeuCode as a reliable proteomic method for deciphering in vivo biology. Display omitted •NeuCode is an accurate quantitative proteomics method of reducing in vivo labeling time•Bap1 deletion perturbs multiple metabolic pathways in pancreas and liver•Histones and mitochondrial membrane proteins are hyperubiquitinated in Bap1ko livers•Liver-specific Bap1ko mice are perinatal lethal with metabolic defects Baughman et al. use NeuCode in vivo labeling for multiplexed quantitation in mouse tissues. Using this approach, coupled with multiple genetically engineered mouse models, they demonstrate a role for Bap1 in maintaining metabolic homeostasis in liver and pancreas.