HOW COME?: Are parts of a potato toxic?

It's spring, and everything's starting to green up, including those potatoes that you left on the kitchen counter. So you put them away in a cool, dark place. But when you pull them out later to make dinner, you find that their green-tinged skin is now studded with cute little sprouts.

Green potatoes with eyes? Unfortunately, it's tossing time for the spuds. Potatoes with a green tint are called "sunburned" because the verdigris color blooms in a reaction with light. When you peel a light-exposed raw potato, you may find a tinge of green a half-inch deep.

The green comes from the chemical chlorophyll, the pigment coloring grass and leaves, peas and broccoli. Chlorophyll is nontoxic, so why waste a green potato?

Think of the green as a marker for chemicals unseen. Sunlight contains both visible and ultraviolet light, and both trigger the production of chemicals called glycoalkaloids on and near a potato's surface. (Indoor lighting can also ruin potatoes. Fluorescents trigger more chlorophyll and alkaloid formation than incandescent bulbs, turning some potatoes green before they leave the supermarket.)

Alkaloids are part of a plant's defense system, fending off fungi, bacteria, viruses, insects and hungry animals. A half-dug-up potato, partially exposed to the sun, will begin to turn green, and its alkaloid production will ramp up. Solanine and a chemical called chaconine are a potato's chemical arsenal. The chemicals are natural insecticides, and like man-made insecticides, nerve toxins.

The glycoalkaloids can cause stomach pain, diarrhea and intestinal bleeding, as well as convulsions, muscle weakness, difficulty breathing, paralysis, coma and even death.

For humans, eating 1 mg. of solanine per pound of body weight can produce symptoms of toxicity, and 2 mg. to about 2.7 mg per pound can be fatal. So a 150-pound person could experience solanine poisoning if he consumed about 150 mg. of the chemical. But tiny amounts of solanine are safe, and according to scientists, the average 150-pound person consumes no more than about 13 mg a day.

A small, fresh, un-green potato naturally contains about 1 to 5 mg. Most solanine is found in the peel, which can be removed or left uneaten. And yes, potato eyes are indeed poisonous: Since they are actively growing, the sprouting roots are especially high in solanine, containing more than 40 mg in each ounce.

To keep potatoes from turning green and making extra alkaloids, store them in a dark, cool place. Throw out old, spongy potatoes; their solanine content may have risen by more than 700 percent. If a potato sports a few sprouts but is still firm, it's OK to cut out the eyes and use the potato. However, if a cooked potato tastes bitter (alkaloids' signature taste), ditch it.