High density lipoprotein metabolism in the horse (Equus caballus).

The article presents a study on the metabolism of high density lipoprotein (HDL), specifically focusing on the enzyme called Apolipoprotein A-I dependent lecithin:cholesterol acyl transferase (LCAT), in horses. The study found no major changes in LCAT activity related to breed, sex, or feeding status, and identified a positive correlation between the levels of HDL and triglyceride in Shetland ponies, but no connection with the activities of LCAT or other related enzymes.

Investigation of Apolipoprotein A-I Dependent LCAT Activity

• The study primarily focuses on understanding the metabolism of HDL in horses, and particularly the role played by the enzyme Apolipoprotein A-I dependent lecithin:cholesterol acyl transferase (LCAT). This enzyme is critical in the process of HDL metabolism, as it converts free cholesterol into cholesteryl ester, which is then stored in the core of the HDL particle.
• In their research, the scientists identified the presence of LCAT activity in what they term as ‘lipoprotein deficient plasma’ (LPDP) in horses, meaning plasma from which all lipoproteins had been removed except for apolipoproteins.

Uniformity of LCAT activity

• The researchers established that there was no variation in LCAT activity based on breed or sex of the horse. This means that all horses, regardless of their breed or sex, show a consistent level of LCAT activity.
• Furthermore, the researchers found that this activity did not change postprandially, or after the horse had eaten. This suggests that LCAT activity is constant and not influenced by feeding status.
• Interestingly, the researchers found that there was no significant cholesteryl ester transfer activity present in the horses’ LPDP, indicating that the process of LDL cholesterol moving to HDL does not seem to occur in these animals to a notable extent.
• Attempts to identify an inhibitor of cholesteryl ester transfer through hydrophobic interaction chromatography were unsuccessful, suggesting that horses do not produce a natural inhibitor of this process.

Correlation with Lipid Levels in Shetland Ponies

• The study found a positive correlation between levels of HDL and triglycerides in the plasma of 12 Shetland ponies. Though higher triglyceride levels are typically associated with health problems in humans, the researchers do not indicate any similar pattern in horses.
• However, they found no correlation between the levels of these lipids and the activities of LCAT, lipoprotein lipase, or hepatic lipase. These findings suggest that the presence and functioning of these enzymes are not directly related to the concentration of lipids in horse plasma.