I've been in Japan for over 20 years, but only brewing for about two. I'll try to give what little insight I have.
Remember that Sapporo, Kirin, Asahi, etc. are not beers. They are companies who make beer -- a reasonably wide range of beers. I assume that what people refer to as "Sapporo" is their Black Label (黒ラベル), Kirin would be Ichiban Shibori (一番搾り) and Asahi would be Super Dry (スーパードライ). Sorry for nitpicking.
Kirin's Ichiban Shibori, as
@Ki-ri-n posted, is made from 100% malt. Black Label and Super Dry, and even Kirin Lager, contain up to 33% adjuncts (rice, corn and starch). Under Japanese tax law, a beer must contain at least 67% barley malt to be called a beer. Anything less than that would make it
happoshu (発泡酒), which sells better than beer in terms of home consumption. Beer is taxed at a much higher rate, though I've read that the government is looking to even things out.
Happoshu is almost never served in bars and restaurants, and it's not exported, as far as I know, as I've never seen it sold outside of Japan. Interestingly, since wheat malt is considered an adjunct, something like a witbier (Sapporo's White Belg comes to mind), is in the lower tax range, and sells for 40% less than the macro lagers.
The Japanese macro lagers have very light hop flavors. I've never bothered to try Orion (from Okinawa), but after reading
@Steven Barrett's post, I'll have to give it a try. Regarding Sorachi Ace hops, I tried a can of Sapporo's Sorachi 1984, and it was simply yucky. The dill was just overpowering. I can't say I've ever noticed that flavor/aroma in Sapporo Black Label. They probably put some in there, but not a lot.
The only malt that's available to homebrewers in Japan is imported malt. My understanding is that barley farmers are contractually obligated to sell all of their harvests to whichever brewery they're affiliated with, so I couldn't tell you about the protein content.
If anyone has any specific questions I'll do my best to answer them.