Cytotoxic and Antioxidant Properties and Profile of Active Compounds in Kale and Lupine Sprouts Supplemented with γ-Polyglutamic Acid During Sprouting

γ-Polyglutamic acid (γ-PGA) is a biodegradable and non-toxic biopolymer with numerous potential applications in agriculture, food, and health sciences due to its antioxidant and antimicrobial properties. This study evaluated the cytotoxic and antioxidant properties of kale (Brassica oleracea var. sabellica) and lupine (Lupinus luteus) sprouts supplemented with different concentrations of γ-PGA. The sprouts were cultivated for various durations (6–10 days), and their bioactive compound profiles were analyzed using HPLC with a PDA 100 UV-VIS detector. Antioxidant activity was assessed via DPPH and FRAP assays, while cytotoxicity was tested against cancer and normal colon cell lines. Results demonstrated that kale sprouts had significantly higher antioxidant activity compared to lupine, with the highest levels observed in kale sprouts supplemented with 0.01% γ-PGA on day 8 (DPPH: 63.6 ± 0.4 μM TEAC/g dw; FRAP: 181.8 ± 2.1 μM/Fe2+/g dw). In contrast, supplementation of lupine sprouts with γ-PGA showed mixed effects, with antioxidant activity depending on concentration and cultivation duration (DPPH in the range 6.5 ± 0.2 ÷ 12.4 ± 0.2 μM TEAC/g dw; FRAP in the range 14.3 ± 0.4 ÷ 25.2 ± 0.9 μM/Fe2+/g dw). Cytotoxicity assays revealed that neither kale nor lupine extracts were toxic to normal colon cells (approx. 100% of alive cells), suggesting selectivity in their action, but fortification with γ-PGA resulted in a weaker or even unfavorable effect on the cytotoxic activity of the examined sprouts. The findings highlight the potential of γ-PGA to enhance the bioactive properties of sprouts, although its effects are influenced by species and cultivation conditions. These results provide a foundation for developing functional foods and sustainable agricultural practices.