What is the primary substrate for the enzyme ALT?

The primary substrate for the enzyme alanine aminotransferase (ALT) is alanine. ALT catalyzes the transamination reaction between alanine and α-ketoglutarate to form pyruvate and glutamate. This reaction plays a significant role in nitrogen metabolism and the biosynthesis of amino acids.

In clinical contexts, the measurement of ALT activity is crucial for assessing liver function, as elevated levels can indicate liver damage or disease. By being directly involved in the conversion of alanine to pyruvate, ALT utilizes alanine as its primary substrate, confirming that option D is the correct choice.