Abstract
Streptococcus thermophilus 510 was investigated as n potential source of $\beta$-galactosidase. Optimum cultural conditions for maximum enzyme production were 0.5% loctose as carbon source, initial pH 7.0, 37 $^{\circ}C$, and 18 hours of cultivation. The enzyme was purified to homogeneity by ammonium sulfate fractionation, protamine sulfate precipitation, Sephadex G-200 gel filtration, and DEAE-Sephadex A-50 ion exchange chromntography. The purified enzyme exhibited an optimum pH at 1.0, and an optimum temperature of 5$0^{\circ}C$. Metal ions such as Mn$^{2+}$ and $K^+$, dithiothreitol, and 2-mercaptoethanol stimulated $\beta$-galactosidase activity. Ethylenediamine tetraacetic add, 8-hydroxyquinoline, Hg$^2+$, Zn$^{2+}$, Co$^{2+}$, $Ca^{2+}$, and galactose were inhibitory. The $K_m$ and V$_{max}$ for o-nitrophenyl $\beta$-D-galactopyranoside were 1.25mM and 88.50$\mu$moles/min.mg protein, respectively. The molecular weight was estimated to be 520,000, and the amino acid composition indicated relatively high contents of glutamic acid, aspartic acid, leucine, and valine.
Streptococcus thermophilus 510으로부터 $\beta$-galactosidase의 생성조건은 탄소원으로 0.5% lactose를 첨가한 배지에서 초기 pH7.0, 배양온도 37$^{\circ}C$, 배양기간 18시간이었다. 배양여액으로부터 $\beta$-galactosidase를 ammonium sulfate 분획, 핵산의 제거, Sephadex G-200 gel filtration 및 DEAE-Sephadex A-50 ion exchange chromatography 등의 4단계 정제과정을 거쳐 정제한 결과 18배 정제되어 단일 단백질로 분리되었다. 정제효소의 활성 최적온도는 5$0^{\circ}C$, 최적 pH는 7.0이었고, 효소활성이 Mn$^{2+}$, $K^+$과 같은 금속이온과 dithiothreitol, 2-mercaptoethanol에 의해 촉진되었고, Hg$^{2+}$, $Zn^{2+}$, Co$^{2+}$, $Ca^{2+}$, EDTA, 8-hydroxyquinoline, galactose 등에 의해 저해되었다. 효소의 분자량이 520,000, 합성기질인 ONPG에 대한 $K_{m}$ 은 1,25mM, V$_{max}$는 88.50 $\mu$mole/min.mg protein이었고, 주종 아미노산은 glutainic acid, aspartic acid, leucine 및 valine이었다.
