However, while APT2 and 3 have much higher affinities for CKs than does APT1, APT1
has almost a 10-fold higher Vmax for each CK as compared with the other isoforms.
Thus the parameter Vmax/KM, which is an estimate of catalytic potential of an enzyme, predicts that the three APT isoforms are very similar in their utilization of these CK substrates in vitro (M Allen, W Qin, F Moreau, BA Moffatt, submitted).
Isolation of mutants deficient in APT2 or APT3 activities and direct measurement of the absolute levels of CKs and their route of synthesis in wild type and the apt1-3 mutant will be required to address whether any of these APTs actually contribute to CK interconversion in vivo.