Fig. 5.

Enzyme kinetics of the ubiquitous ${\alpha }_1$ isoform and the neuron specific ${\alpha }_3$ isoform. (a) Kinetics data taken from Zahler et al.¹⁵ Normalized pump rates for ${\alpha }_1$ (black line) and ${\alpha }_3$ (gray line) as a function of sodium concentrations. (b) The 50% intracellular sodium affinity value ($K_{\mathrm{m}}$) is 12 mM for ${\alpha }_1$ and 33 mM for ${\alpha }_3$, and the Hill coefficient is 3.5 for ${\alpha }_1$ and 6.8 for ${\alpha }_3$. Pump rates for both isoforms scaled to reflect the difference in maximum rate, so that the neuron specific isoform ${\alpha }_3$ (gray line) has a 10 times higher maximum turnover rate than ${\alpha }_1$ (black line). Additionally, the summed pump rate from combining (equal amounts of) the isoforms is also shown (black-dashed line).