= D. C. Klein
{c}
<D. C. Klein> coauthored a 1970 study connecting a <circadian rhythm> to the activity of <serotonin N-acetyltransferase> in the rat <pineal gland>. With <J. L. Weller>, <D. C. Klein> found activity more than fifteen times higher at night than during the day.<Indole metabolism in the pineal gland: a circadian rhythm in N-acetyltransferase by D. C. Klein and J. L. Weller>[Abstract: day/night enzyme-activity comparison]{ref}
The rhythm persisted in complete darkness and in blinded animals, while constant illumination suppressed it. These comparisons distinguished an internally maintained rhythm from a response requiring the eye to detect each transition between day and night.<Indole metabolism in the pineal gland: a circadian rhythm in N-acetyltransferase by D. C. Klein and J. L. Weller>[Abstract: darkness, blinding and constant-light comparisons]{ref}
The study also tested regulation in isolated preparations: <norepinephrine>, rather than <serotonin>, regulated the measured activity under those conditions. Combining the time course with an intervention on the biochemical system connected timing to a particular step associated with <melatonin> synthesis.<Indole metabolism in the pineal gland: a circadian rhythm in N-acetyltransferase by D. C. Klein and J. L. Weller>[Abstract: in-vitro regulation and relation to melatonin rhythm]{ref}
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