Fig. 7.

Kinetic properties of SNF-5 interaction with substrates. (A) Amino acid saturation profiles. The specified amino acids were applied in staircase concentration increments: 0.011<0.033<0.11<0.33<1.1<3.3<10 mmol l⁻¹ prepared in 7.2 pH buffered 98 mmol l⁻¹ Na⁺ medium. Graphs show only up to the 3 mmol l⁻¹ concentrations of the specified amino acids. (B) Na⁺ saturation profiles with linear and logarithmic scale, respectively. The specified amino acids were applied at 1 mmol l⁻¹ concentrations in media containing variable concentrations of Na⁺. Stepwise, these concentrations were: 1<3.3<10<33<98 mmol l⁻¹, where Na⁺ was substituted with equimolar concentrations of N-methyl-d-glucamine (NMDG⁺). Data points are mean ± s.e.m. current responses normalized to maximum current in individual oocytes, for N>3 different oocytes. The data were extrapolated to fit a three-parameter sigmoidal Hill approximation f=I_maxI^η/(K_0.5^η +I^η) without parametric constraints of Michaelis–Menten (η=1). Estimated affinities (K_0.5) and apparent order of translocation events (η, Hill constant) for amino acid and Na⁺ substrates are summarized in Tables 2 and 3, respectively.