Kinetic and immunologic evidence for a complete gene block in myoadenylate deaminase deficiency

Muscle biopsies from patients with myoadenylate deaminase deficiency (mADD) have been evaluated kinetically and immunologically to ascertain the origin of residual enzyme activity. Kinetic evaluation employed 5 mM AMPS/1 mM AMP ratios, which were 0.7-0.8 for the human muscle isozyme, but 0-0.25 for the isozyme(s) of all other human blood cells and tissues examined. Of 14 control biopsies, 13 showed a ratio greater than 0.60 (one gave 0.47) regardless of the enzyme specific activity, while all 14 mADD biopsies showed a ratio less than 0.24, suggesting that a fetal muscle isozyme and/or blood cell isozyme were responsible for the residual activity. Confirmation was provided by rabbit antisera to purified human muscle AMP deaminase. These antisera fully precipitate the isozyme from crude human muscle biopsy homogenates, regardless of fiber-type composition, and cross-react effectively with the muscle isozyme of Rhesus monkeys and thoroughbred horses, but are inactive toward the isozymes of all other human blood cells and tissues examined. Of 18 mADD homogenates tested, 14 showed less than 20% reactivity with the antisera, at levels that precipitated 10 x more enzyme in control specimens. The residual activity in most cases of mADD must therefore arise from some source other than normal AMP deaminase. To evaluate the possibility of a single common determinant, 9 mADD homogenates were tested for soluble immune complexes. Seven of the 9 then showed 20-42% reactivity, suggesting that part of their residual activity may be due to an isozyme sharing one antigenic determinant with the normal muscle isozyme. Competitive antigen binding was used to assess whether catalytically inactive AMP deaminase was present in mADD. The method was demonstrated effective in identifying spontaneously inactivated purified enzyme and alkaline-inactivated crude enzyme. Nevertheless, homogenates from 17 mADD cases failed to produce more than 14% activation, under conditions in which 63-99% activation was expected. Triton X-100 extracts of homogenate residues of 11 mADD cases were also tested, in a search for insoluble antigen; none produced significant competition. The evidence thus indicates that most cases of mADD are due to a complete gene block, with total absence of all normal muscle AMP deaminase protein.