What property of alkaline phosphatase and γ-glutamyltransferase makes them useful for detecting defects in bile flow?

The property that makes alkaline phosphatase and γ-glutamyltransferase useful for detecting defects in bile flow is that these enzymes are present in bile ductules. When there is an obstruction or defect in bile flow, such as in cholestasis, these enzymes can leak into the bloodstream, resulting in elevated serum levels.

Alkaline phosphatase is found in significant amounts in the liver, bone, intestines, and placenta, but its elevation is especially indicative of cholestatic conditions when isolated to liver function. Similarly, γ-glutamyltransferase is also associated with cholestasis and can indicate liver disease when levels are increased. Because these enzymes are closely related to the biliary tree, their activity in the blood serves as a marker for bile duct obstruction or liver conditions affecting bile flow.

Other properties of these enzymes, like their involvement in bile salt synthesis, bilirubin conjugation, or their presence on the basal surface of hepatocytes, do not directly relate to their diagnostic utility for bile flow defects in the same way as their localization within bile ductules. Hence, the relevance lies in their secretion and activity concerning the health of the bile ducts and the liver's ability to transport bile.