@hdeol, line 15 talks about how serotonin is a derivative of tryptophan, which means tryptophan can be converted into serotonin. Since line 17 is talking about the "rate of conversion" it must be referencing the conversion of tryptophan into serotonin. The passage goes into how insulin stimulates the uptake of amino acids, but that's not a conversion issue and therefore can't be what the passages is referencing as the "rate of conversion" in line 17.
