(Na⁺, K⁺)- ATPase kinetics in Macrobrachium pantanalense: highlighting intra- and interspecific variation within the Macrobrachium amazonicum complex

The Macrobrachium amazonicum complex is composed of at least the Macrobrachium amazonicum and Macrobrachium pantanalense species, with the latter described from specimens originally identified as part of an endemic M. amazonicum population in the Brazilian Pantanal region. While there may be a reproductive barrier between these two Macrobrachium species, both are phylogenetically close, with small genetic distance. However, there is currently no available biochemical information of Macrobrachium pantanalense (Na⁺, K⁺)-ATPase. Here, we report the kinetic characteristics of the gill (Na⁺, K⁺)-ATPase in two populations of M. pantanalense from Baiazinha Lagoon (Miranda, MS, Brazil) and Araguari River (Uberlândia, MG, Brazil), and compare them with Macrobrachium amazonicum populations from the Paraná-Paraguay River Basin. (Na⁺, K⁺)-ATPase activities were 67.9 ± 3.4 and 93.3 ± 4.1 nmol Pi min^-1 mg^-1 protein for the Baiazinha Lagoon and Araguari River populations, respectively. Two ATP hydrolyzing sites were observed for the Araguari River population while a single ATP site was observed for the Baiazinha Lagoon shrimps. Compared to the Araguari River population, a 3-fold greater apparent affinity for Mg²⁺ and Na⁺ was estimated for the Baiazinha Lagoon population, but no difference in K⁺ affinity and ouabain inhibition was seen. The kinetic differences observed in the gill (Na⁺, K⁺)-ATPase between the two populations of M. pantanalense, compared with those of various M. amazonicum populations, highlight interspecific divergence within the Macrobrachium genus, now examined from a biochemical perspective.