Fig 1.
Comparative titration curves of HCl (control), polyamines, and oligolysines.
Before titration all solutions were acidified to a value beneath the lowest pKa of the reactants. When comparing (1A) the polyamines putrescine (Put), spermidine (Spd), and spermine (Spm), the only compound that started buffering below a value of 7.4 was Spm. So, this is the only polyamine (PA), which to some degree may serve to stabilize the intracellular pH value within the physiological range. All the oligolysines (tri-lys, Lys3; tetra-lys, Lys4; penta-lys, Lys5) buffer the first time around pH 6.0 (alpha amino group) and secondly again above 8.0 far outside the physiological range (1B). In addition, the two molecules with four positive charges (Spm and Lys3) are compared directly (1C).
Table 1.
The pKs values of putrescine, spermidine, spermine, di-lysine, and tri-lysine as given here are taken from the investigations referenced.
They are in good agreement with our data provided in the present manuscript.
Fig 2.
Comparative interaction curves polyamines and tri-lysine with DNA.
In the present study we measured changes in the UV absorption at 260 nm (E260) of herring sperm DNA in the presence of PAs or tri-lysine (Lys3). Addition of PUT, the smallest PA with only two charged groups, did not produce any alteration in the E260 of the DNA (Fig 2A). In contrast, SPD and SPM appear to interact with the DNA, as both PAs strongly heighten E260 values. With SPD the increase is highest at 0.5 mM, followed by a steep decrease in extinction. This decrease, however, must not be interpreted as conformational change but is simply the result of DNA precipitation at higher SPD concentrations (Fig 2B). With SPM the maximum increase occurs at 1 mM (Fig 2C), followed by precipitation. Again, the elevated E260 values at SPM concentrations above 1 mM may be due to turbidity produced by the precipitate. Addition of Lys3, which like SPD bears three positively charged groups, results in no increase at E260 (Fig 2D), even at concentrations up to 45 mM. Thus, only the longer PAs like SPD and SPM, and neither PUT nor Lys3 are able to interact and produce conformational changes in double stranded DNA molecules.
Fig 3.
Indirect contribution of PAs to intracellular pH homeostasis.
PAs may indirectly contribute to the stabilization of the intracellular pH value, as their biosynthesis includes several steps, which are thought to increase the intracellular pH value. The present scheme is an extract of our earlier one (20: Rieck et al, 2022, Fig 2) representing the biosynthesis of PAs from ornithine. Here, we focus on reactions, which release H+-ions (highlighted by red boxes) into the cytoplasm. Viewing this scheme, however, one should bear in mind that the biosynthesis of S-adenosylmethionine (methionine plus ATP releases three inorganic phosphates) should produce more H+-ions than had been consumed before.