The activity of sucrose-phosphate synthase (SPS) in 9-day-old barley (Hordeum vulgare L.) primary leaves was measured over a 24-hour period. Extractable enzyme activity was constant in the light, decreased 50 to 60% during the first one-half hour of darkness, and then returned to full activity before the start of the normal light period. Decreases of SPS activity in the dark were fully reversed by less than 10 minutes of illumination. In contrast to results with barley, the measurable activity of SPS in soybean, spinach, and pea leaves was unchanged during the first hour of darkness. Changes of SPS activity in barley primary leaves were stable upon gel filtration. The exact biochemical mechanism responsible for the enzyme activity changes in barley leaf extracts is unknown. The above findings support the suggestion by de Fekete (1973 Eur J Biochem, 10: 73-80) that SPS is controlled by posttranslational protein modification. These results are discussed in relation to the regulation of photosynthetic sucrose metabolism.
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