The latency of peroxisomal catalase in terms of effectiveness factor for pancreatic and glioblastoma cancer cell lines in the presence of high concentrations of H2O2: Implications for the use of pharmacological ascorbate in cancer therapy

Abstract

Previous research has identified variation in cancer cell line response to high levels of extracellular H₂O₂ (eH₂O₂) exposure. This directly contributes to our understanding cellular efficacy of pharmacological ascorbate (P-AscH^-) therapy. Here we investigate the factors contributing to latency of peroxisomal catalase of a cell and the importance of latency in evaluating cell exposure to eH₂O₂. First, we develop a mathematical framework for the latency of catalase in terms of an effectiveness factor, η_eff, to describe the catalase activity in the presence of high levels of eH₂O₂. A simplified relationship emerges, [Formula: see text] when m_pr_p/D_ij≪1, where m_p,r_p, and [Formula: see text] are the experimentally determined peroxisome permeability, average peroxisome radius, and the pseudo first-order reaction rate constant, respectively. [Formula: see text] is the catalase concentration in the peroxisome and k₂=1.7x10⁷M^-1s^-1. Next, previously published parameters are used to determine the latency effect of the cell lines: normal pancreatic cells (H6c7), pancreatic cancer cells (MIA PaCa-2), and glioblastoma cells (LN-229, T98G, and U-87), all which vary in their susceptibility to exposure to high eH₂O₂. The results show that effectiveness is not significantly different except for the most susceptible, MIA PaCa-2 cell line, which is higher when compared to all other cell lines. This result is counterintuitive and further implies that latency, as a single parameter, is ineffective in forecasting cell line susceptibility to P-AscH^- therapy equivalent eH₂O. Thus, further research remains necessary to identify why cancer cells vary in susceptibility to P-AscH^- therapy.

Fig 1.. The latency of the catalase in the peroxisome is associated with the peroxisome membrane permeability, m_p, and the overall average catalase activity within the peroxisome.

With respect to the overall activity, each catalase molecule in solution, represented on the left-hand side, has access to the same concentration of H₂O₂ in the well-mixed system. However, within the peroxisome (right), the catalase near the surface of the organelle will encounter higher concentration of H₂O₂ than the catalase near the center. This is depicted by the larger H₂O₂ arrow in the outer dashed surface compared to the smaller arrow for the inner dashed line. Since the overall reaction rate is a linear function of the concentration of H₂O₂, the reaction rate of the catalase near the center is reduced. The effectiveness factor takes this into consideration and directly correlates with observed latency.

Fig 2.. The relationship of the effectiveness factor in terms of cellular properties expressed in terms of the Biot number, Bi_p, and the Thiele modulus, ϕ_p.

The Biot number is a function of peroxisome radius, r_p, and peroxisome membrane permeability, m_p. The Thiele modulus is a function r_p of and catalase concentration, $C_{ca{t}_p}$. All cases have Bi_p ≪ 1 indicating that mass transfer resistance is highly significant, and, for a given Thiele modulus, increasing Bi_p increases effectiveness. Smaller ϕ_p result in higher effectiveness.

Fig 3.. Comparison of the calculated effectiveness to cell lines used in ascorbate dosing studies.

These results demonstrate a trend; however, only the effectiveness between the highly susceptible MIA PaCa-2 and the other cell lines are statistically different. This observed difference is counterintuitive to the expectation that increased latency (low effectiveness) could result in increased susceptibility. References for figure: *Chen et al. (2005)[21], **Chen et al. (2008)[22], ***Du et al. (2010)[1].