Table 2.
Comparison of the characteristics of the NKA and NKAa4 isoform in species in which most of the studies have been performed.
| Human | Rat | Mouse | Bull | |
|---|---|---|---|---|
| NKA isoform composition in sperm | HKAa1, NKAa4, NKAl31, NKAl33 | HKAa1, NKAa4, NKAl31, NKAl33 | HKAa1, NKAa4, NKAl31, NKAl33 | NKAa1, NKAa3, NKAa4, NKAl31, NKAl32, NKAl33 |
| Main localization of NKA isoforms in sperm | Flagellum, midpiece | Flagellum, midpiece | Flagellum, midpiece | Head (NKAa1 equatorial region; NKAa3 in post-equatorial region) |
| Chromosome location of NKAa4 | 1q23.2 | 13q24 | 1H3 | 3 |
| NKAa4 homology comparison between species | 83% (rat), 83% (mouse), 87% (bull) | 83% (human), 94% (mouse), 82% (bull) | 83% (human), 94% (rat), 83% (bull) | 87% (human), 82% (rat), 83% (mouse) |
| Enzymatic properties of NKAa4 relative to other NKA isoforms within the same species | High affinity for Na+, low affinity for K+ | High affinity for Na+, low affinity for K+, intermediate affinity for ATP | High affinity for Na+, low affinity for K+, intermediate affinity for ATP | Non-determined |
| Ouabain IC50 | High sensitivity to ouabain (nM) | High sensitivity to ouabain (nM) | High sensitivity to ouabain (nM) | Not exactly determined |
| Role in sperm | Motility and hypermotility | Motility and hypermotility | Motility and hypermotility | Motility, capacitation, and acrosomal reaction |