Which enzyme has the highest circulating concentration?

• A. Ceruloplasmin
• B. Amylase
• C. Lipase
• D. Creatine kinase

Answer: (A)

Circulating enzyme-like proteins vary a lot in blood, and one with a major presence in plasma stands out because it’s produced in large amounts and serves a transport role. Ceruloplasmin is a major copper-binding protein in the blood and also has ferroxidase activity. It is released continuously by the liver, so its baseline level in plasma is relatively high compared with other enzymes listed.

In contrast, amylase and lipase are digestive enzymes that are secreted into the gut and only appear in measurable serum amounts under certain conditions; their normal circulating levels are comparatively low. Creatine kinase is an intracellular enzyme abundant in muscle tissue; while it can rise significantly in muscle injury, its baseline serum concentration is lower than that of ceruloplasmin.

So, the substance with the highest circulating concentration among these is ceruloplasmin.