Animal experiment:

Mice[2]C57BL/6J pun/pun mice are used in the study. Pregnancy is timed by checking for vaginal plugs. Noon of the day of discovery is counted as 0.5 days post coitum (d.p.c.). Similarly, the time of birth of a litter is timed with the noon of discovery counted as 0.5 days post-partum (d.p.p.). Pregnant dams are given free access to drinking water supplemented by either 2 mM L-Buthionine-(S,R)-sulfoximine (BSO), 20 mM BSO, 2 mM BSO and 20 mM NAC, 20 mM NAC or unsupplemented water for 18 days from 0.5 to 18.5 d.p.c. The pH of supplemented water is as follows: 6.88, 20 mM BSO; 3.37, 2 mM BSO; 2.65, 2 mM BSO plus 20 mM NAC; and 2.58, 20 mM NAC. The pH of regular water used in our facility is ~4. To determine the DNA deletion frequency, 20-day-old offspring (23 mice in the control group and 16-17 mice per exposure group) are sacrificed to visualize eye-spots (DNA deletions) in their RPE[2].

L-Buthionine-(S,R)-sulfoximine is a cell-permeable, potent, fast acting and irreversible inhibitor of g-glutamylcysteine synthetase and depletes cellular glutathione levels. The IC50 value of L-Buthionine-(S,R)-sulfoximine on melanoma, breast and ovarian tumor specimens are 1.9 μM, 8.6 μM, and 29 μM, respectively.

L-Buthionine-(S,R)-sulfoximine (BSO: 50 μM) treatment for 48 hr results in a 95% decrease in ZAZ and M14 melanoma cell line GSH levels, and a 60% decrease in GST enzyme activity. GST-π protein and mRNA levels are significantly reduced in both cell lines[1]. L-Buthionine-(S,R)-sulfoximine (BSO) induces oxidative stress in a cell by irreversibly inhibiting g-glutamylcysteine synthetase, an essential enzyme for the synthesis of glutathione (GSH)[2].

The average number of eye-spots (mean±SEM) is 5.36±0.29 (n=46), 7.79±0.45 (n=34) and 8.78±0.61 (n=32) in untreated controls, 2 mM L-Buthionine-(S,R)-sulfoximine (BSO) and 20 mM BSO treated mice, respectively. The 2 mM BSO treatment results in ~30% more eye-spots, and the 20 mM treatment results in 40% more eye-spots compared with untreated mice. It is showed that BSO causes an elevated frequency of DNA deletions during mouse development. BSO treatment reduced GSH concentration in mouse fetuses by 55% and 70% at 2 mM and 20 mM BSO doses, respectively, compared to untreated mice. Co-treatment with 2 mM BSO and 20 mM NAC depleted GSH to a similar extent as 2 mM BSO, consistent with the function of BSO to inhibit the g-GCS enzyme indispensable for GSH synthesis. Like GSH, cysteine levels dropped following BSO treatment[2].

[1]. Fruehauf JP, et al. Selective and synergistic activity of L-S,R-buthionine sulfoximine on malignant melanoma is accompanied by decreased expression of glutathione-S-transferase. Pigment Cell Res. 1997 Aug;10(4):236-49. [2]. Reliene R, et al. Glutathione depletion by buthionine sulfoximine induces DNA deletions in mice. Carcinogenesis. 2006 Feb;27(2):240-4.