transferase are Eli Lilly, Takeda Pharmaceutical Company, and Boehringer Ingelheim. Pharmaceutical development One medical use of ghrelin O-acyl transferase inhibitors has been in the preclinical treatment of Prader-Willi Syndrome, a rare genetic disorder resulting in early-onset diabetes. In 2018, Rhythm Pharmaceuticals announced a licensing agreement with Takeda Pharmaceutical to develop an orally administered ghrelin O-acyl transferase inhibitor in order to decrease the amount of active ghrelin in individuals suffering from Prader-Willi Syndrome. Preclinical research has shown that this drug prevented weight gain with positive pharmacological and safety profiles. Additionally, ghrelin O-acyl transferase inhibition has been used in clinical trials in the United Kingdom in studying appetite regulation of post-surgical bariatric patients. The study measured, among other things, the effect of decreased ghrelin levels on gut hormone, adipokine, and cytokine levels. Chemical Properties Structure-activity studies have demonstrated that the ‘GSSF’ N-terminal sequence of ghrelin is essential to ghrelin O-acyl transferase substrate recognition. While the enzyme will accept sequences with a threonine reside in place of serine, catalytic activity is reduced in these cases. Ghrelin O-acyl transferase is also poorly stereospecific and is able to recognize and acylate both stereoisomers of serine. The enzymatic activity of GOAT is also altered when treated with cysteine-modifying substances. Interestingly, the enzyme is relatively insensitive to pH changes in the range of 6-9. Kinetic studies also suggest that GOAT activity is consistent with Michaelis-Menten kinetics in the acylation of ghrelin. One paper proposes that bioavailability of the GOAT substrates represent the rate-limiting step of the acyl transfer reaction and not the chemical process itself. Biochemical Assays Several biochemical assays have been developed to study the activity of the enzyme under in vitro conditions. The most common is the use of an amino acid and radioactive isotope tags to monitor the formation of reaction products. Also widespread is the use of indirect methods to observe ghrelin O-acyl transferase function such as the detection of acylated ghrelin products, which indicate enzyme activity. These assays are based on high-throughput ELISA based techniques. There has also been published literature describing protocols for ghrelin O-acyl transferase function expression and enrichment in insect cells, the use of high performance liquid chromatography assays for acylation activity, and the use of fluorescent protein labels. Metabolic Function In humans, the ghrelin O-acyl transferase function enzyme post-translationally modifies an inactive form of ghrelin, known as proghrelin, on an N terminus serine residue. This function is essential