The papaya leaf contains beta-carotene, calcium, carpaine, fats, flavonols, niacin, papain, tannins, and vitamin C (in higher concentration in the leaf than in the fruit). The leaf, unlike the fruit, is not a source of the protein-dissolving enzyme papain, but the latex (sap) in the leaf stem is. Papain remains in leaf preparations that have been dried over low heat, but it may be destroyed in products that are dried at high heat.

Bring the water and leaves to the boil and simmer (without a lid) until the water is reduced by half.
Strain the liquid and bottle in glass container. The concentrate will keep in the refrigerator for three to four days. If it becomes cloudy it should be discarded. The recommended dosage in the original recipe is 50 ml three times a day.
Health benefits for treatment of cancer, papaya leaf juice is claimed to have reversed cancer in many people living on the Gold Coast in Australia.
There is also news that papaya leaf juice can help reduce dengue fever.
Papaya leaf juice aids in the breakdown and metabolism of protein, exhibits anti-ulcer activity and helps to relieve indigestion. Papaya leaf contains the powerful proteolytic enzymes, papain and chymopapain, which digest proteins, small peptides, amides and esters. Their activity extends to carbohydrate and fat, and they are said to be more effective than naturally- occurring proteases like pepsin and trypsin. Since stomach ailments are often the direct result of indigestion, papaya leaf juice may help prevent and remedy these by increasing digestive processes, as the digestive properties of papain are well established.
Papaya Leaf Juice