As stated in the text, adenosine deaminase deficiency can be treated by injection of the stabilized enzyme.

Question:

As stated in the text, adenosine deaminase deficiency can be treated by injection of the stabilized enzyme. Why might this treatment be effective, while injection of the missing enzyme is ineffective in other conditions, such as HGPRT deficiency or a deficiency of one of the mitochondrial deoxyribo-nucleoside kinases?