Lactate Biology: Subcellular Routing and Chemical Form Define Function.

Lactate has undergone a major conceptual shift, from a glycolytic waste product to a circulating metabolic currency and, more recently, a multifunctional regulator of physiology. It serves as a mitochondrial fuel, an epigenetic modifier via protein lactylation, a ligand for the G-protein-coupled receptor HCAR1, and a precursor for endocrine-like N-lactoyl amino acids. This convergence raises a central question: how can a single metabolite support such distinct roles without functional conflict? We propose that the resolution lies not in lactate concentration alone but in two complementary organizing principles: subcellular routing and chemical form. Transporter localization, enzyme compartmentalization, donor formation, and metabolic competition bias lactate toward distinct biochemical fates, while conversion into chemically distinct intermediates─including lactyl-CoA, lactoyl-glutathione, d-lactate, and N-lactoyl amino acids─further constrains the biological outcomes that lactate can support. Lactate's fate is influenced by the cellular compartments it accesses, a process constrained by specific monocarboxylate transporters at the plasma and mitochondrial membranes, isoform-specific localization of lactate dehydrogenases, and compartmentalized enzymatic machinery that converts lactate into distinct biochemical donors. Mitochondrial oxidation, protein lactylation, extracellular signaling, and N-lactoyl amino acid synthesis should therefore not be viewed as parallel consequences of elevated lactate concentration. Instead, they represent interconnected metabolic fates that draw from shared lactate pools and are influenced by compartmental access and local enzymatic context. Here, we integrate evidence from metabolism, epigenetics, and signaling into a spatial framework in which lactate function depends on where it is routed. In this view, lactate is not a promiscuous metabolite but a compartmentalized intermediate whose biological effects are shaped by spatial context. We further distinguish between established, emerging, and speculative aspects of this compartmentalized view to highlight key gaps and prioritize future experimental testing.