Hydrolysis of urea by Ureaplasma urealyticum generates a transmembrane potential with resultant ATP synthesis

Abstract

When urea is added to Ureaplasma urealyticum, it is hydrolysed internally by a cytosolic urease. Under our measuring conditions, and at an external pH of 6.0, urea hydrolysis caused an ammonia chemical potential equivalent to almost 80 mV and, simultaneously, an increase in proton electrochemical potential (h) of about 24 mV with resultant de novo ATP synthesis. Inhibition of the urease with the potent inhibitor flurofamide abolished both the chemical potential and the increase of Ap such that ATP synthesis was reduced to -5% of normally obtained levels. Uncouplers of electrochemical gradients had little or no effect on these systems. The electrochemical parameters and ATP synthesis were measured similarly at three other external pH values. Any change in Ap was primarily via membrane potential (A*), and the level of de novo ATP synthesis was related to the increase in Ap generated upon addition of urea and more closely to the ammonia chemical potential.Although the organisms lack an effective mechanism for internal pH homeostasis, they maintained a constant ApH. The data reported are consistent with, and give evidence for, the direct involvement of a chemiosmotic mechanism in the generation of around 95% of the ATP by this organism. Furthermore, the data suggest that the ATP-generating system is coupled to urea hydrolysis by the cytosolic urease via an ammonia chemical potential.

Molecular Biology and Pathogenicity of Mycoplasmas

“…The dependence of ureaplasmas on urea for growth has led to the hypothesis that intracellular urea hydrolysis and the resulting intracellular accumulation of ammonia/ammonium ions is coupled to ATP synthesis through a chemiosmotic type of mechanism (356). Experimental support for the generation of a transmembrane potential, with resultant ATP synthesis through the ureaplasmal F O F 1 -type ATPase, first provided by Romano et al (383) was more recently extended and confirmed by Smith et al (428). At an external pH of 6.0, the pH optimum for ureaplasma growth in vitro, urea hydrolysis generated an ammonia chemical potential equivalent to almost 80 mV and, simultaneously, an increase in proton electrochemical potential (⌬p) of about 24 mV with resultant de novo synthesis of ATP (428).…”

“…Experimental support for the generation of a transmembrane potential, with resultant ATP synthesis through the ureaplasmal F O F 1 -type ATPase, first provided by Romano et al (383) was more recently extended and confirmed by Smith et al (428). At an external pH of 6.0, the pH optimum for ureaplasma growth in vitro, urea hydrolysis generated an ammonia chemical potential equivalent to almost 80 mV and, simultaneously, an increase in proton electrochemical potential (⌬p) of about 24 mV with resultant de novo synthesis of ATP (428). When the external pH of the growth medium reaches about 8.1, due to ammonia released from the cells, the intracellular pH rises to 8.6 and urease activity ceases (as shown by the pH activity profile of the ureaplasmal urease).…”

“…Ureaplasmas are unique among the mollicutes in possessing a very potent urease. Although protein and gene analysis of the ureaplasmal urease complex has shown that it resembles other procaryotic ureases in subunit structure and composition, the specific activity of the ureaplasmal urease is much higher and was estimated to exceed that of jack bean urease by about 100-fold (50,316,428,452). More importantly, ureaplasmas appear to be unique among procaryotes in requiring urea for growth (356).…”

“…When the external pH of the growth medium reaches about 8.1, due to ammonia released from the cells, the intracellular pH rises to 8.6 and urease activity ceases (as shown by the pH activity profile of the ureaplasmal urease). ATP generation is blocked at this alkaline pH, leading to the abrupt and steep decline of growth characterizing the ureaplasmal growth curve (356,428). Inhibition of the urease by flurofamide abolished both the chemical potential and the increase of ⌬p, such that ATP synthesis decreased to ϳ5% of the normally obtained levels (428).…”

“…ATP generation is blocked at this alkaline pH, leading to the abrupt and steep decline of growth characterizing the ureaplasmal growth curve (356,428). Inhibition of the urease by flurofamide abolished both the chemical potential and the increase of ⌬p, such that ATP synthesis decreased to ϳ5% of the normally obtained levels (428). Lansoprazole, another specific urease inhibitor, also inhibited ATP synthesis and ureaplasma growth (320).…”