Abstract Body (Do not enter title and authors here): INTRODUCTION/BACKGROUND
Using plasma proteomic profiling, we recently identified aminoacylase-1 (ACY1) as a top biomarker associated with metabolic dysfunction, including obesity, dyslipidemia, steatotic liver disease, and incident type-2 diabetes (T2D). ACY1 is highly expressed in the liver, where it hydrolyzes N-acetyl-amino acids to free amino acids, but its physiological role in metabolism remains unexplored.

HYPOTHESIS
ACY1 may play a functional role in regulating hepatic and systemic metabolic homeostasis.

METHOD/APPROACH
Metabolic phenotyping was performed on C57BL/6N-Acy1^em1 knockout (KO) mice (RRID:MMRRC_046467-UCD) using indirect calorimetry, functional assays, and high-resolution respirometry in isolated liver mitochondria.

RESULTS
ACY1 KO animals exhibited no gross reproductive, developmental, or behavioral abnormalities. Deletion of ACY1 markedly increased hepatic and plasma N-acetyl-amino acid levels (e.g., N-acetyl-glutamine: 23.5-fold increase in liver compared to wild-type (WT) controls, p=4.6E-5; 7.5-fold increase in plasma, p=1.2E-6). Compared to WT controls, KO mice developed greater body weight (34.3 ± 1.4 vs. 28.2 ± 1.0 g, p=2.3E-3), increased fat mass (15.9 ± 1.2 vs. 9.0 ± 1.1 g measured by MRI, p=5.4E-4), impaired glycemic control (intraperitoneal glucose tolerance AUC of 347 ± 8.3 vs 278 ± 9.2, p<1E-4), and hepatic steatosis (35.4 ± 4.4 vs. 9.6 ± 1.3 % Oil Red O positive area, p=8E-4) after 24 weeks on a high-fat diet (60 kcal% fat). At 8 weeks of age, prior to overt differences in body weight, KO mice exhibited significantly reduced total body energy expenditure without differences in food intake, physical activity, or respiratory exchange ratio. These early systemic changes were accompanied by significantly decreased state 2 and state 3 respiration in isolated hepatic mitochondria, independent of changes in overall mitochondrial content.

CONCLUSION
These findings identify ACY1 as a previously unrecognized regulator of metabolic homeostasis, linking its loss to reduced energy expenditure, impaired mitochondrial function, and increased susceptibility to diet-induced metabolic dysfunction. Further studies are ongoing to elucidate the mechanisms by which ACY1 and its downstream pathways regulate systemic metabolism, and to explore their potential as therapeutic targets in metabolic disease.