Angiotensin I-converting enzyme (ACE) plays a major role in blood pressure regulation and fluid and electrolyte homeostasis in both humans and horses. While ACE enzyme concentrations in plasma are highly stable in an individual, there is a high level of interindividual variability. In humans, ACE levels in blood obtained from athletes have been correlated with aptitude for endurance sports, that is, lower mean plasma ACE activity is found in the fastest endurance athletes. Additionally in humans, a genetic polymorphism (I/D) in the ACE gene has been discovered and this polymorphism accounts for almost 50% of the variation in ACE activity in plasma.
In horses, research results have demonstrated an inverse correlation between ACE activity in the blood and the distance successfully raced by racehorses. Costa et al found that exercise training of racehorses produced a gradual increase in ACE enzymatic activity and might warrant the use of ACE as a tool for fitness monitoring, but that the enzyme level per-se was not a good predictor of overall performance in endurance horses. As yet, none of this work has been applied to Thoroughbreds, however, in a multi-species paper published earlier this year, Hamilton, et al completed an extensive study of the sequence and structure of the equine ACE gene and has identified common haplotypes of the gene based on 9 SNPs across a diverse cohort of breeds (Thoroughbreds, Arabians, Standardbreds, Clydesdales and Shires). A further study of the SNPs recognized in this study that is underway now may uncover a functional variant of the ACE gene in horses that affects the enzyme level and biological performance.