HNMT and hyperinsulinemic hypoglycemia, familial, 4: In addition, this is related to our previous findingsregarding age-specific increase in antioxidant and neuroprotectivemetabolites, such as α-tocopherol, carnosine, and l-carnitine, in astrocytic dense brain regions, a possibly compensatingmechanism for elevated oxidative brain activity.42 Another astrocytic enzyme that decreases with aging ishistamine N-methyltransferase (HNMT).7,43 Accordingly, HNMT deficiency may contribute to the depletion of N-methylhistamine.44 Simultaneously,we observed an aging-correlated increase in His, which may furthersupport the diminished HNMT action.44