Nelson O E, Chourey P S, Chang M T
Department of Genetics, University of Wisconsin, Madison, Wisconsin.
Plant Physiol. 1978 Sep;62(3):383-6. doi: 10.1104/pp.62.3.383.
Starch granule preparations from the endosperm tissue of all waxy maize (Zea mays L.) mutants tested have low and approximately equal capability to incorporate glucose from adenosine diphosphate glucose into starch. As the substrate concentration is reduced, however, the activity of waxy preparations relative to nonmutant increases until, at the lowest substrate concentration utilized (0.1 muM), the activity of the waxy preparations is nearly equal to that of the nonmutant preparation. The apparent K(m) (adenosine diphosphate glucose) for starch granule preparations from wx-C/wx-C/wx-C endosperms was 7.1 x 10(-5) M, which is compared to 3 x 10(-3) M for preparations from nonwaxy endosperms. Starch granule preparations from three other waxy mutants of independent mutational origin have levels of enzymic activity approximately equal to wx-C at a given substrate concentration giving rise to similar apparent K(m) estimates. We conclude that there is in maize endosperm starch granules a second starch granule-bound glycosyl transferase, whose presence is revealed when mutation eliminates activity of the more active glucosyl transferase catalyzing the same reaction.
对所有测试的糯玉米（Zea mays L.）突变体胚乳组织的淀粉颗粒制剂而言，其从二磷酸腺苷葡萄糖中摄取葡萄糖并将其掺入淀粉的能力较低且大致相等。然而，随着底物浓度的降低，糯玉米制剂相对于非突变体的活性增加，直至在所用的最低底物浓度（0.1 μM）时，糯玉米制剂的活性几乎与非突变体制剂的活性相等。来自wx-C/wx-C/wx-C胚乳的淀粉颗粒制剂的表观K(m)（二磷酸腺苷葡萄糖）为7.1×10⁻⁵ M，相比之下，来自非糯性胚乳的制剂的表观K(m)为3×10⁻³ M。来自另外三个独立突变起源的糯玉米突变体的淀粉颗粒制剂，在给定底物浓度下的酶活性水平与wx-C大致相等，从而得出类似的表观K(m)估计值。我们得出结论，在玉米胚乳淀粉颗粒中存在第二种与淀粉颗粒结合的糖基转移酶，当突变消除催化相同反应的活性更高的葡萄糖基转移酶的活性时，该酶的存在就会显现出来。
