Final answer:
Lipoprotein lipase is the enzyme down-regulated during sedentary behaviors, leading to lower HDL levels. This enzyme is vital for breaking down triglycerides into fatty acids and thus affects lipid metabolism.
Step-by-step explanation:
The enzyme that is down-regulated during sedentary behaviors, leading to a reduction in blood levels of high-density lipoprotein (HDL), is lipoprotein lipase. Lipoprotein lipase plays a crucial role in lipid metabolism by breaking down triglycerides in lipoproteins like chylomicrons and very-low-density lipoproteins (VLDL) into free fatty acids and glycerol, allowing the uptake of fatty acids by muscle and adipose tissues for energy use or storage. When this enzyme's activity is decreased, the clearance of triglycerides from the plasma is slowed, causing a decrease in HDL, which is known as the "good" cholesterol transporter, since it is associated with a lower risk of cardiovascular diseases.